##language:zh ''' 含有章节索引的中文 文章模板 ''' -- ZoomQuiet [<>] ## The Whole Python FAQ Python常遇問題集全文 ## Last changed on Mon Nov 11 22:48:25 2002 CET 最新更新日期 2002年11月11日(星期一) 22:48:25 ## (Entries marked with ** were changed within the last 24 hours; entries marked with * were changed within the last 7 days.) <> ## -------------------------------------------------------------------------------- ## 1. General information and availability = 一般資訊及如何取得 = ## -------------------------------------------------------------------------------- ## 1.1. What is Python? == Python 是什麼? == ## Python is an interpreted, interactive, object-oriented programming language. It incorporates modules, exceptions, dynamic typing, very high level dynamic data types, and classes. Python combines remarkable power with very clear syntax. It has interfaces to many system calls and libraries, as well as to various window systems, and is extensible in C or C++. It is also usable as an extension language for applications that need a programmable interface. Finally, Python is portable: it runs on many brands of UNIX, on the Mac, and on PCs under MS-DOS, Windows, Windows NT, and OS/2. ## To find out more, the best thing to do is to start reading the tutorial from the documentation set (see a few questions further down). ## See also question 1.17 (what is Python good for). ## Edit this entry / Log info / Last changed on Mon May 26 16:05:18 1997 by GvR Python 是一種即譯式的, 互動的, 面向對象的編程語言. 它包含了模組式的操作, 異常處理, 動態資料形態, 十分高層次的動態資料結構, 以及類別的使用. Python 揉合了簡單的語法和強大的功能. 它擁有眾多的編程介面以操控系統平台以及各類函數庫. 利用 c 和 c++ 可以對它進行擴充. 個別的應用軟件如果需要有一個可程序化介面也可以利用它來做為擴展語言用. 最後, Python 的可移植度非常高: 它可以在許多的 Unix 類平台上運行, 在 Mac, MS-DOS, 視窗 Windows, Windows NT, OS/2, BeOS, 以至 RISCOS 上都有相關的 Python 版本. 想知道更多, 最好是從 Python 教學資源中的 "入門篇" 看起. (參考下幾條問題) 請一併參看第 1.17 條問題. (Python 可以用在什麼地方) ## -------------------------------------------------------------------------------- ## 1.2. Why is it called Python? == 為什麼取名叫 Python? == ## Apart from being a computer scientist, I'm also a fan of "Monty Python's Flying Circus" (a BBC comedy series from the seventies, in the -- unlikely -- case you didn't know). It occurred to me one day that I needed a name that was short, unique, and slightly mysterious. And I happened to be reading some scripts from the series at the time... So then I decided to call my language Python. ## By now I don't care any more whether you use a Python, some other snake, a foot or 16-ton weight, or a wood rat as a logo for Python! ## Edit this entry / Log info / Last changed on Thu Aug 24 00:50:41 2000 by GvR Python 的意思是蟒蛇, 但和這套語言的命名沒有關係. Python 的作者除了是一位電腦計算學家外, 也是 BBC 喜劇片 "Monty Python's Flying Circus" (七十年代一套受歡迎的片集)的影迷. 作者在替他的新語言改名時要找一個短的,獨一無二的並且帶些神秘感的名字, 於是他決定了把這個語言命名為 Python. Python 沒有自己的商標, 你可以隨便用蟒蛇, 或是別的什麼蛇屬, 無論你喜歡它是兩尺長還是 16 吨重, 甚至你用一隻小老鼠都沒有所謂. (譯注: 市面上有幾本關於 Python 的專書都用了上面所說的動物, 因此作者在這裏澄清.) ## -------------------------------------------------------------------------------- ## 1.3. How do I obtain a copy of the Python source? == 怎樣取得 Python 的源程式碼? == ## The latest Python source distribution is always available from python.org, at http://www.python.org/download. The latest development sources can be obtained via anonymous CVS from SourceForge, at http://www.sf.net/projects/python . ## The source distribution is a gzipped tar file containing the complete C source, LaTeX documentation, Python library modules, example programs, and several useful pieces of freely distributable software. This will compile and run out of the box on most UNIX platforms. (See section 7 for non-UNIX information.) ## Older versions of Python are also available from python.org. ## Edit this entry / Log info / Last changed on Tue Apr 9 17:06:16 2002 by A.M. Kuchling 最新的 Python 源程式碼一定可以在 python.org 上取得. 可到 http://www.python.org.download 下載. 至於最近開發中的版本則可以經 SourceForge 透過 CVS 以 anonymous 形式取得, 網址是 http://www.sf.net/projects/python 取得的源碼為 gzip 了的 tar 檔案. 當中有全部的 C 程式碼, LaTeX 格式的說明文件, Python 的函式庫模組, 範例和一些有用的自由軟件. 在大部份 Unix 類平台上能直接編譯安裝. (在非 Unix 平台上的使用方法, 請看第 7 節中的問題. ## -------------------------------------------------------------------------------- ## 1.4. How do I get documentation on Python? == 怎樣取得有關 Python 的說明文件 == ## All documentation is available on-line, starting at http://www.python.org/doc/. ## The LaTeX source for the documentation is part of the source distribution. If you don't have LaTeX, the latest Python documentation set is available, in various formats like postscript and html, by anonymous ftp - visit the above URL for links to the current versions. ## PostScript for a high-level description of Python is in the file nluug-paper.ps (a separate file on the ftp site). ## Edit this entry / Log info / Last changed on Wed Jan 21 12:02:55 1998 by Ken Manheimer 所有的說明文件可以到網上拿取, 有關資料可先從 http://www.python.org/doc/ 找起. 說明文件的 LaTeX 文檔已包括在源碼包中. 如果你沒有 LaTeX 可用, 在上面的網址裏可找到已做好的各種格式的現成文件 (如 html 或 postscript). 有關 Python 的中文文件不是很多, 已翻譯的有 "教學文件 (Tutorial)", "語言參考手冊" (Language Reference), "深入 Python" (Dive into Python). 這些可在 http://www.chinesepython.org/cgi_bin/moingb.cgi/ 上取得. 你也可以購買市面上關於 Python 的中文書. ## -------------------------------------------------------------------------------- ## 1.5. Are there other ftp sites that mirror the Python distribution? == 有沒有其他網站可以下載 Python 各發佈版 == ## The following anonymous ftp sites keep mirrors of the Python distribution: ## USA: ## ftp://ftp.python.org/pub/python/ ## ftp://gatekeeper.dec.com/pub/plan/python/ ## ftp://ftp.uu.net/languages/python/ ## ftp://ftp.wustl.edu/graphics/graphics/sgi-stuff/python/ ## ftp://ftp.sterling.com/programming/languages/python/ ## ftp://uiarchive.cso.uiuc.edu/pub/lang/python/ ## ftp://ftp.pht.com/mirrors/python/python/ ## ftp://ftp.cdrom.com/pub/python/ ## Europe: ## ftp://ftp.cwi.nl/pub/python/ ## ftp://ftp.funet.fi/pub/languages/python/ ## ftp://ftp.sunet.se/pub/lang/python/ ## ftp://unix.hensa.ac.uk/mirrors/uunet/languages/python/ ## ftp://ftp.lip6.fr/pub/python/ ## ftp://sunsite.cnlab-switch.ch/mirror/python/ ## ftp://ftp.informatik.tu-muenchen.de/pub/comp/programming/languages/python/ ## Australia: ## ftp://ftp.dstc.edu.au/pub/python/ ## Edit this entry / Log info / Last changed on Wed Mar 24 09:20:49 1999 by A.M. Kuchling 以下 ftp 站點可以下載 Python: 美國: － ftp://ftp.python.org/pub/python/ － ftp://gatekeeper.dec.com/pub/plan/python/ － ftp://ftp.uu.net/languages/python/ － ftp://ftp.wustl.edu/graphics/graphics/sgi-stuff/python/ － ftp://ftp.sterling.com/programming/languages/python/ － ftp://uiarchive.cso.uiuc.edu/pub/lang/python/ － ftp://ftp.pht.com/mirrors/python/python/ － ftp://ftp.cdrom.com/pub/python/ 歐洲: － ftp://ftp.cwi.nl/pub/python/ － ftp://ftp.funet.fi/pub/languages/python/ － ftp://ftp.sunet.se/pub/lang/python/ － ftp://unix.hensa.ac.uk/mirrors/uunet/languages/python/ － ftp://ftp.lip6.fr/pub/python/ － ftp://sunsite.cnlab-switch.ch/mirror/python/ － ftp://ftp.informatik.tu-muenchen.de/pub/comp/programming/languages/python/ 澳大利亞: － ftp://ftp.dstc.edu.au/pub/python/ ## -------------------------------------------------------------------------------- ## 1.6. Is there a newsgroup or mailing list devoted to Python? == 有沒有專門開給 Python 的新聞組和電郵列表 == ## There is a newsgroup, comp.lang.python, and a mailing list. The newsgroup and mailing list are gatewayed into each other -- if you can read news it's unnecessary to subscribe to the mailing list. To subscribe to the mailing list (python-list@python.org) visit its Mailman webpage at http://www.python.org/mailman/listinfo/python-list ## More info about the newsgroup and mailing list, and about other lists, can be found at http://www.python.org/psa/MailingLists.html. ## Archives of the newsgroup are kept by Deja News and accessible through the "Python newsgroup search" web page, http://www.python.org/search/search_news.html. This page also contains pointer to other archival collections. ## Edit this entry / Log info / Last changed on Wed Jun 23 09:29:36 1999 by GvR 有一個新聞組, comp.lang.python, 和一個電郵列表. 它們之間是聯通的 -- 如果你能讀得到新聞組則你不需要申請加入電郵表. 要加入電郵表(python-list@python.org) 請到以下 Mailman 的網站: http://www.python.org/mailman/listinfo/python-list (Mailman 也是用 Python 寫成的). 除此之外還有許多其也的 Python 電郵列表, 請到 http://www.python.org/psa/MailingLists.html 參觀. Deja News 保留了有關新聞組的貼文內容, 你可以到 "Python 新聞組搜尋"站, http://www.python.org/search/search_news.html. 該網頁上還可連結到別的文件庫去. ## -------------------------------------------------------------------------------- ## 1.7. Is there a WWW page devoted to Python? == 有沒有專門的網站是關於 Python 的 == ## Yes, http://www.python.org/ is the official Python home page. ## Edit this entry / Log info / Last changed on Fri May 23 14:42:59 1997 by Ken Manheimer 有的, http://www.python.org/ 是 Python 的官方網站. ## -------------------------------------------------------------------------------- ## 1.8. Is the Python documentation available on the WWW? == 在網上是不是可以找到 Python 的說明文件. == ## Yes. Python 2.0 documentation is available from http://www.pythonlabs.com/tech/python2.0/doc/ and from http://www.python.org/doc/. Note that most documentation is available for on-line browsing as well as for downloading. ## Edit this entry / Log info / Last changed on Tue Jan 2 03:14:08 2001 by Moshe Zadka 是. 在 http://www.python.org/doc/ 中可以找到所有的官方說明文件. 大部分可供即時瀏覽和下載. ## -------------------------------------------------------------------------------- ## 1.9. Are there any books on Python? == 有沒有關於 Python 的書. == ## Yes, many, and more are being published. See the python.org Wiki at http://www.python.org/cgi-bin/moinmoin/PythonBooks for a list. ## You can also search online bookstores for "Python" (and filter out the Monty Python references; or perhaps search for "Python" and "language"). ## Edit this entry / Log info / Last changed on Mon Aug 5 19:08:49 2002 by amk 有許多, 更多的正在印制中. 請留意 python.org 的 Wiki -- http://www.python.org/cgi-bin/moinmoin/PythonBooks 上的書單. 至於中文書則市面上才剛剛出現了幾本. (譯注: 請有心人加上資料.) ## -------------------------------------------------------------------------------- ## 1.10. Are there any published articles about Python that I can reference? == 有什麼可供引用的 Python 文章. == ## If you can't reference the web site, and you don't want to reference the books (see previous question), there are several articles on Python that you could reference. ## Most publications about Python are collected on the Python web site: ## http://www.python.org/doc/Publications.html ## It is no longer recommended to reference this very old article by Python's author: ## Guido van Rossum and Jelke de Boer, "Interactively Testing Remote ## Servers Using the Python Programming Language", CWI Quarterly, Volume ## 4, Issue 4 (December 1991), Amsterdam, pp 283-303. ## Edit this entry / Log info / Last changed on Sat Jul 4 20:52:31 1998 by GvR 如果你不想用 Python 的網站, 也不打算用書 (看前面的問題), 在 Python 的網站上有些相關的文章你可以參考: http://www.python.org/doc/Publications.html ## -------------------------------------------------------------------------------- ## 1.11. Are there short introductory papers or talks on Python? == 有沒有比較短的介紹 Python 的資料或是演講. == ## There are several - you can find links to some of them collected at http://www.python.org/doc/Hints.html#intros. ## Edit this entry / Log info / Last changed on Fri May 23 15:04:05 1997 by Ken Manheimer 也有一些 -- 請到主站點找有關的連結 http://www.python.org/doc/Hints.html#intros. ## -------------------------------------------------------------------------------- ## 1.12. How does the Python version numbering scheme work? == Python 的版本編號是用哪種方法來取名. == ## Python versions are numbered A.B.C or A.B. A is the major version number -- it is only incremented for really major changes in the language. B is the minor version number, incremented for less earth-shattering changes. C is the micro-level -- it is incremented for each bugfix release. See PEP 6 for more information about bugfix releases. ## Not all releases have bugfix releases. Note that in the past (ending with 1.5.2), micro releases have added significant changes; in fact the changeover from 0.9.9 to 1.0.0 was the first time that either A or B changed! ## Alpha, beta and release candidate versions have an additional suffixes. The suffix for an alpha version is "aN" for some small number N, the suffix for a beta version is "bN" for some small number N, and the suffix for a release candidate version is "cN" for some small number N. ## Note that (for instance) all versions labeled 2.0aN precede the versions labeled 2.0bN, which precede versions labeled 2.0cN, and those precede 2.0. ## As a rule, no changes are made between release candidates and the final release unless there are show-stopper bugs. ## You may also find version numbers with a "+" suffix, e.g. "2.2+". These are unreleased versions, built directly from the CVS trunk. ## See also the documentation for sys.version, sys.hexversion, and sys.version_info. ## Edit this entry / Log info / Last changed on Mon Jan 14 06:34:17 2002 by GvR Python 的版本號形式是 A.B.C 或是 A.B. 其中 A 是主版本編號 -- 只有當語言本身有很大的轉變時才會調大. B 是版本副號, 當程式改變較少時用以區別新舊版本. C 則是小號, 當發行除蟲版本時便用. 有關除蟲版本的資料請參閱 PEP 6. Alpha (測試), Beta (調試), release (發布) 建議推出的版本都有其另外的編號後綴. 測試版用 aN, 調試版用 bN, 發布版用 cN, 其中 N 代表一個不太大的數. 按一般的規定, 除非在建議發布版有極大的缺陷, 否則建議發布和正式發布版間不應有任何改動. 有時你會遇到加有 '+' 後綴的版本編號, 例如 "2.2+". 這些版本不曾正式發布過, 它們是從 CVS 軟庫中直接編譯而來的. 你也可以參看 sys.version, sys.hexversion 和 sys.version_info 的說明, 裏面有些額外的資料. ## -------------------------------------------------------------------------------- ## 1.13. How do I get a beta test version of Python? == 我如何取得測試版的 Python. == ## All releases, including alphas, betas and release candidates, are announced on comp.lang.python and comp.lang.python.announce newsgroups, which are gatewayed into the python-list@python.org and python-announce@python.org. In addition, all these announcements appear on the Python home page, at http://www.python.org. ## You can also access the development version of Python through CVS. See http://sourceforge.net/cvs/?group_id=5470 for details. If you're not familiar with CVS, documents like http://linux.oreillynet.com/pub/a/linux/2002/01/03/cvs_intro.html provide an introduction. ## Edit this entry / Log info / Last changed on Mon Jun 3 00:57:08 2002 by Neal Norwitz 所有發布的版本, 包括建議為測試, 調試和發布的各版本推出的消息都會在 comp.lang.python 和 comp.lang.python.announce 這兩個新聞組上公告. 這兩個新聞組分別和 python-list@python.org, python-announce@pytyon.org 聯通. 除此之外, 在 Python 的官方網站 http://www.python.org 上也會公告有關消息. 至於 Python 開發中的最新版本可以透過 CVS 取得. 請上 http://sourceforge.net/cvs/?group_id=5470 .如你不熟悉 CVS 的操作方法, 可以閱讀有關的文章, 如 http://linux.oreillynet.com/pub/a/linux/2002/01/03/cvs_intro.html 就有介紹. ## -------------------------------------------------------------------------------- ## 1.14. Are there copyright restrictions on the use of Python? == 使用 Python 都有哪些版權上的限制. == ## Hardly. You can do anything you want with the source, as long as you leave the copyrights in, and display those copyrights in any documentation about Python that you produce. Also, don't use the author's institute's name in publicity without prior written permission, and don't hold them responsible for anything (read the actual copyright for a precise legal wording). ## In particular, if you honor the copyright rules, it's OK to use Python for commercial use, to sell copies of Python in source or binary form, or to sell products that enhance Python or incorporate Python (or part of it) in some form. I would still like to know about all commercial use of Python! ## Edit this entry / Log info 很難說有何限制. 拿到源碼後隨便你幹些啥, 唯一的條件是你保留原來的版權聲明, 並在你寫的有關文件中展示那些聲明. 同時, 未得到作者所屬的機構的書面同意, 不要在宣傳時引用它的名字, 也不要代他們作出任何形式的承諾 (具體的法律字眼請參閱版權聲明的原文). 有一點要說清楚的是, 如果你尊守上述的要求, 那就算你利用 Python 作商業用途, 包括售賣它的源碼或是執行檔, 或者是改進了的 Python 以及在產品中加入 Python (部分或是全部). 另外, 如果你的把它用作商業盈利, 請通知作者因為他也很有興趣知道. ## -------------------------------------------------------------------------------- ## 1.15. Why was Python created in the first place? == 從一開始,為什麼要創造 Python 這個語言. == ## Here's a very brief summary of what got me started: ## I had extensive experience with implementing an interpreted language in the ABC group at CWI, and from working with this group I had learned a lot about language design. This is the origin of many Python features, including the use of indentation for statement grouping and the inclusion of very-high-level data types (although the details are all different in Python). ## I had a number of gripes about the ABC language, but also liked many of its features. It was impossible to extend the ABC language (or its implementation) to remedy my complaints -- in fact its lack of extensibility was one of its biggest problems. I had some experience with using Modula-2+ and talked with the designers of Modula-3 (and read the M3 report). M3 is the origin of the syntax and semantics used for exceptions, and some other Python features. ## I was working in the Amoeba distributed operating system group at CWI. We needed a better way to do system administration than by writing either C programs or Bourne shell scripts, since Amoeba had its own system call interface which wasn't easily accessible from the Bourne shell. My experience with error handling in Amoeba made me acutely aware of the importance of exceptions as a programming language feature. ## It occurred to me that a scripting language with a syntax like ABC but with access to the Amoeba system calls would fill the need. I realized that it would be foolish to write an Amoeba-specific language, so I decided that I needed a language that was generally extensible. ## During the 1989 Christmas holidays, I had a lot of time on my hand, so I decided to give it a try. During the next year, while still mostly working on it in my own time, Python was used in the Amoeba project with increasing success, and the feedback from colleagues made me add many early improvements. ## In February 1991, after just over a year of development, I decided to post to USENET. The rest is in the Misc/HISTORY file. ## Edit this entry / Log info / Last changed on Fri May 23 00:06:23 1997 by GvR 以下的作者的回憶: 我在 CWI 的 ABC (譯按: 另一種即譯式語言) 開發組工作時, 學到很多設計程式語言方面的事並且得到許多經驗. Python 多方面繼承了這一套語言的特性, 如縮排式的分段方法和高層次的數據類等.(當然,在語言的實現細節上兩者很不一樣.) 我對於 ABC 的某些地方很不以為然, 當然, 它有很多地方是讓人喜愛的. 對這些我不喜歡的地方我發現並不能透過擴充 ABC 來滿足我的要求 -- 事實上, 難以擴充正正是 ABC 的最大弱點. 與此同時, 我擁有使用 Modula-2+ 的一些經驗並且和一些 Modula-3 的設計者們溝通過 (也讀了 M3 的報告書). M3 中的異常處理和某些別的句法是 Python 中有關功能的出處. 當時我在 CWI 的 Amoeba 分散操作系統小組工作. 我們需要一個更好的辦法來做系統管理工作, 我們並不想用 C 寫程式或是用 Bourne shell 寫腳本, 因為 Amoeba 有它自己一套調用系統的介面不是用 Bourne shell 可以輕易存取的. 而我在 Amoeba 的異常處理方面的經驗便我很清楚的意識到在程式語言中包含錯誤處理功能的重要程度. 於是我的腦海出現了一套和 ABC 句法類似同時可以呼叫 Amoeba 系統功能的即譯語言, 它應該能滿足我的要求. 我知道如果我把它設計成只能是 Amoeba 專用的語言並不是聰明的做法, 我需要的是一套更普遍, 並能多方面擴充的語言. 我有大量時間可以幹自己想做的活, 於是我決定開始實現我的構想. 在接下來的一年中, 雖然我依然在用工餘時間來做開發, Python 已經頗為成功的應用到 Amoeba 計劃中了. 同事們給了許多意見, 這些都轉變成早期對 Python 的改良和完善工作. 到了 1991 年的 2 月, 就在經過短短一年的開發工作, 我決定把 Python 張貼到 USENET 上去讓更多人使用. 後來發生的事情很多人都知道, 都寫在 Misc/HISTORY 這個文件中了. ## -------------------------------------------------------------------------------- ## 1.16. Do I have to like "Monty Python's Flying Circus"? == 我是不是一定要喜歡 "Month Python's Flying Circus" 這套片子. == ## No, but it helps. Pythonistas like the occasional reference to SPAM, and of course, nobody expects the Spanish Inquisition ## The two main reasons to use Python are: ## - Portable ## - Easy to learn ## The three main reasons to use Python are: ## - Portable ## - Easy to learn ## - Powerful standard library ## (And nice red uniforms.) ## And remember, there is no rule six. ## Edit this entry / Log info / Last changed on Wed May 28 10:39:21 1997 by GvR 不. ## -------------------------------------------------------------------------------- ## 1.17. What is Python good for? == Python 可以用在什麼地方. == ## Python is used in many situations where a great deal of dynamism, ease of use, power, and flexibility are required. ## In the area of basic text manipulation core Python (without any non-core extensions) is easier to use and is roughly as fast as just about any language, and this makes Python good for many system administration type tasks and for CGI programming and other application areas that manipulate text and strings and such. ## When augmented with standard extensions (such as PIL, COM, Numeric, oracledb, kjbuckets, tkinter, win32api, etc.) or special purpose extensions (that you write, perhaps using helper tools such as SWIG, or using object protocols such as ILU/CORBA or COM) Python becomes a very convenient "glue" or "steering" language that helps make heterogeneous collections of unrelated software packages work together. For example by combining Numeric with oracledb you can help your SQL database do statistical analysis, or even Fourier transforms. One of the features that makes Python excel in the "glue language" role is Python's simple, usable, and powerful C language runtime API. ## Many developers also use Python extensively as a graphical user interface development aide. ## Edit this entry / Log info / Last changed on Sat May 24 10:13:11 1997 by Aaron Watters Python 可以用在很多埸合. 當你需要大量的動態調整, 要容易使用, 功能強大並且富彈性的情況, Python 可以發揮很好的功效. 在處理文字方面, Python 的核心功能 (不需任何擴充庫的情況下) 較起別的編程語言更為易用而速度相當. 這使 Python 在需要處理字符串的工作 (包括系統管理, CGI 編程) 上成為高效的語言. 當 Python 加上它的標準擴充庫 (如 PIL, COM, Numeric, oracledb, kjbuckets, tkinter, win32api, 等) 或是特別擴充庫 (你自己寫的, 或是用 SWIG 做的, 或是利用 ILU/COBRA/COM 的)後, 它可以變成一個很好的 "膠合語言" 或者叫 "指導語言". 就是指把異質的編程環境和互不相關的軟件功能整合處理的工具. 例如把 Numeric 和 oracledb 透過 Python 結合起來你就可以替你的資料庫數據做統計分析, 甚至是做譜分析. Python 的簡單, 易用以及方便而強大的 C 擴展介面使它成為一種十分優秀的 "膠合語言". 許多開發人員也在寫圖像用戶介面時廣泛地採用了 Python. ## -------------------------------------------------------------------------------- ## 1.18. Can I use the FAQ Wizard software to maintain my own FAQ? == 我可以在這個版面中加入我自己的問題嗎 == ## Sure. It's in Tools/faqwiz/ of the python source tree. ## Edit this entry / Log info / Last changed on Fri Mar 29 06:50:32 2002 by Aahz (譯按: 當然可以. 凡是有關 Python 的般性問題都可自由地放上來. 但請注意特殊問題或技巧或是和官方問題集差別太大的問題請貼到本站其他的問題集中.) ## -------------------------------------------------------------------------------- ## 1.19. Which editor has good support for editing Python source code? == 編輯 Python 程式有哪些文字編輯器比較好用. == ## On Unix, the first choice is Emacs/XEmacs. There's an elaborate mode for editing Python code, which is available from the Python source distribution (Misc/python-mode.el). It's also bundled with XEmacs (we're still working on legal details to make it possible to bundle it with FSF Emacs). And it has its own web page: ## http://www.python.org/emacs/python-mode/index.html ## There are many other choices, for Unix, Windows or Macintosh. Richard Jones compiled a table from postings on the Python newsgroup: ## http://www.bofh.asn.au/~richard/editors.html ## See also FAQ question 7.10 for some more Mac and Win options. ## Edit this entry / Log info / Last changed on Mon Jun 15 23:21:04 1998 by Gvr 在 Unix 上, 首選 Emacs/XEmacs. 它有個專為編寫 Python 程式用的模式, 其設定檔可以在 Python 的源程式碼中找到 (Misc/python-mode.el). 在 XEmacs 的安裝時已包括了這個設定 (至於 FSF Emacs 方面我們正在和他們商量有關的法律細節以便這個設定也可以包括在軟件發布版中.) 這個設定的詳情請 到 http://www.python.org/emacs/python-mode/index.html 其他的選擇也不少, 無論是 Unix, Windows, Micintosh 都有. Richard Jones 根據新聞組的內容綜合了一張表, 到 http://www.bofh.asn.au/~richard/editors.html ## -------------------------------------------------------------------------------- ## 1.20. I've never programmed before. Is there a Python tutorial? == 我從來沒有學過寫程式,有沒有一些入門級的學習材料. == ## There are several, and at least one book. All information for beginning Python programmers is collected here: ## http://www.python.org/doc/Newbies.html ## Edit this entry / Log info / Last changed on Wed Sep 5 05:34:07 2001 by GvR 有幾篇文章, 另外起碼一本書. 所有適合 Python 初學者看的資料都綜合成一個網頁, 到 http://www.python.org/doc/Newbies.html 至於中文的資料, 本站也有一些, 請到 'Python 入門' 頁面. ## -------------------------------------------------------------------------------- ## 1.21. Where in the world is www.python.org located? == www.python.org 的位置在哪裏. == ## It's currently in Amsterdam, graciously hosted by XS4ALL: ## http://www.xs4all.nl ## Thanks to Thomas Wouters for setting this up!!!! ## Edit this entry / Log info / Last changed on Fri Aug 3 21:49:27 2001 by GvR 現時位於阿姆斯特丹, 服務由 XS4ALL 所提供. ## -------------------------------------------------------------------------------- == www.chinesepython.org 的位置在哪裏. == 現時位於香港, 由 NetZide 所提供. ## -------------------------------------------------------------------------------- == Python 正確發音是什麼. == 我聽過有兩種讀法, 皆為雙音節, 首音為 "pi" (圓周率), 尾音為 "夫 on" (on 為英文的 on/off 的 on), 首音為 "pi" (圓周率), 尾音為 "夫 en" (en 為韻母 an/en/in 的 en, 或英文 one/two/three 的 one). ## -------------------------------------------------------------------------------- ## 2. Python in the real world = Python 於真實世界的應用 = ## -------------------------------------------------------------------------------- ## 2.1. How many people are using Python? == 有多少人在使用 Python == ## Certainly thousands, and quite probably tens of thousands of users. More are seeing the light each day. The comp.lang.python newsgroup is very active, but overall there is no accurate estimate of the number of subscribers or Python users. ## Jacek Artymiak has created a Python Users Counter; you can see the current count by visiting http://www.wszechnica.safenet.pl/cgi-bin/checkpythonuserscounter.py (this will not increment the counter; use the link there if you haven't added yourself already). Most Python users appear not to have registered themselves. ## Edit this entry / Log info / Last changed on Thu Feb 21 23:29:18 2002 by GvR 數以千計的人在使用它, 現在可能已經有過萬甚至十萬用戶了. 幾乎每天都有新的用戶加入. 新聞組上的討論也很熱烈 (comp.lang.python), 但事實上並沒有一個確切的統計到底有多少人正在用. Jacek Artymiak 寫了一個 Python 用戶的計數程式, 你可以到 http://www.wszechnica.safenet.pl/cgi-bin/checkpythonuserscounter.py 看看 (瀏覽這個網頁不會另計數器產生變化, 你可以利用網頁上的連結把你自己也算進去). 大部分 Python 用戶好像都沒有登記. ## -------------------------------------------------------------------------------- ## 2.2. Have any significant projects been done in Python? == 有沒有任何大型的軟件是用 Python 完成的? == ## At CWI (the former home of Python), we have written a 20,000 line authoring environment for transportable hypermedia presentations, a 5,000 line multimedia teleconferencing tool, as well as many many smaller programs. ## At CNRI (Python's new home), we have written two large applications: Grail, a fully featured web browser (see http://grail.cnri.reston.va.us), and the Knowbot Operating Environment, a distributed environment for mobile code. ## The University of Virginia uses Python to control a virtual reality engine. See http://alice.cs.cmu.edu. ## The ILU project at Xerox PARC can generate Python glue for ILU interfaces. See ftp://ftp.parc.xerox.com/pub/ilu/ilu.html. ILU is a free CORBA compliant ORB which supplies distributed object connectivity to a host of platforms using a host of languages. 在 CWI (Python 的老家), 我們寫過一個 20,000 行長的軟件用來作為編寫超媒體展示用的書寫環境; 另外一個 5,000 行的多媒體遠程會工具, 和其他許多的小程式. 在 CNRI (Python 的新家), 我們寫了兩個大東西: Grail 是一個全功能的網頁瀏覽器 (http://grail.cnri.reston.va.us), 和 Knowbot 操作環境, 一個給流動程式的分散操作環境. 維珍尼亞大學用 Python 來操控虛擬實境的引擎. Xeros PARC 的 ILU 計劃用 Python 來做 ILU 介面的膠合工具. 請到 ftp://ftp.parc.xerox.com/pub/ilu/ilu.html. ILU 是符合 CORBA 設計的一個免費 ORB, 它為很多很多的操作平台上的許許多多語言提供分散式的物象連結. (譯按: 天啊, 我到底在寫什麼?) ## Mark Hammond and Greg Stein and others are interfacing Python to Microsoft's COM and ActiveX architectures. This means, among other things, that Python may be used in active server pages or as a COM controller (for example to automatically extract from or insert information into Excel or MSAccess or any other COM aware application). Mark claims Python can even be a ActiveX scripting host (which means you could embed JScript inside a Python application, if you had a strange sense of humor). Python/AX/COM is distributed as part of the PythonWin distribution. ## The University of California, Irvine uses a student administration system called TELE-Vision written entirely in Python. Contact: Ray Price rlprice@uci.edu. Mark Hammond, Greg Stein 和其他人把 Python 接進了微軟的 COM 和 ActiveX 架構之中. 意思就是 (別的先不說), Python 現在可以用來寫 ASP (Active Server Page), 或者用來控制 COM 對象. (例如自動從 Excel 或 Access 中抽取或是添入數據, 或是調用任何有 COM 功能的軟件來完成工作). Mark 聲稱甚至可以用 Python 來做 ActiveX 的 scripting host (誇張的說, 你完全可以把 JScript 鑲在一個 Python 應用程式中, 如果你覺得有需要的話.) Python/AX/COM 包含在 PythonWin 的發布套件當中. 加利褔利亞大學的 Irvine 所使用的學生管理系統整個都是由 Python 寫出來的, 它叫做 TELE-Vision. 有關資料可以聯絡 Ray Price rlprice@uci.edu. ## The Melbourne Cricket Ground (MCG) in Australia (a 100,000+ person venue) has it's scoreboard system written largely in Python on MS Windows. Python expressions are used to create almost every scoring entry that appears on the board. The move to Python/C++ away from exclusive C++ has provided a level of functionality that would simply not have been viable otherwise. ## See also the next question. ## Note: this FAQ entry is really old. See http://www.python.org/psa/Users.html for a more recent list. 澳大利亞的墨爾本木球埸 (Melbourne Criket Ground (MCG), 可容納 100,000+ 觀眾) 的計分牌系統是用 MS Windows 加大量的 Python 炮製出來的. 分牌上的每一個數字都是由 Python 所產生的. 利用 Python 加 C++ 而不是純粹的 C++ 提供了前所未見的簡單而強大的功能. 下一個問題的答案也可作為本題的參考. 注意: 這一段內容已經十分老了, 請到 http://www.python.org/psa/Users.html 閱讀更新的資料. ## Edit this entry / Log info / Last changed on Wed Oct 25 13:24:15 2000 by GvR ## -------------------------------------------------------------------------------- ## 2.3. Are there any commercial projects going on using Python? == 是否有用 Python 的商業產品 ? == ## Yes, there's lots of commercial activity using Python. See http://www.python.org/psa/Users.html for a list. ## Edit this entry / Log info / Last changed on Wed Oct 14 18:17:33 1998 by ken 有哇. 有很多都用的不亦樂乎呢. 看 http://www.python.org/psa/Users.html ## -------------------------------------------------------------------------------- ## 2.4. How stable is Python? == Python 有多穩定 ? == ## Very stable. New, stable releases have been coming out roughly every 3 to 12 months since 1991, and this seems likely to continue. ## With the introduction of retrospective "bugfix" releases the stability of the language implementations can be, and is being, improved independently of the new features offered by more recent major or minor releases. Bugfix releases, indicated by a third component of the version number, only fix known problems and do not gratuitously introduce new and possibly incompatible features or modified library functionality. ## Release 2.2 got its first bugfix on April 10, 2002. The new version number is now 2.2.1. The 2.1 release, at 2.1.3, can probably be considered the "most stable" platform because it has been bugfixed twice. ## Edit this entry / Log info / Last changed on Tue Jul 23 10:20:04 2002 by Jens Kubieziel 非常非常穩定. 自 1991 年開始, 每 3 到 12 個月就有一個新並且穩定的版本推出來, 以目前來看, 這情況會一直持續下去. 我們也引進了所謂的 "除蟲版", 就是說每個推出的除蟲版本只會改正發現的錯誤而不會替版本添加新的, 不相容的功能甚式更改函數庫的功能. 這樣保證了版本間穩定和功能互相獨立. 除蟲版的號碼顯示在版本號的第三組編號上. 第 2.2 版的首次除蟲版於 2002 年 4 月 10 日推出, 現在的新版本編號是 2.2.1. 至於 2.1 版, 現在到了 2.1.3, 這個版本可以說是目前為止最穩定的因為它推出了兩次除蟲版. ## -------------------------------------------------------------------------------- ## 2.5. What new developments are expected for Python in the future? == 在未來 Python 會有什麼新的發展 ? == ## See http://www.python.org/peps/ for the Python Enhancement Proposals (PEPs). PEPs are design documents describing a suggested new feature for Python, providing a concise technical specification and a rationale. ## Also, follow the discussions on the python-dev mailing list. ## Edit this entry / Log info / Last changed on Tue Apr 9 17:09:51 2002 by A.M. Kuchling 請看 http://www.python.org/peps/ 上的 Python Enhancement Proposals (PEPs, Python 改良提案). PEPs 中多為對 Python 的建議修改. 它包含簡潔的技術說明的理據. ## -------------------------------------------------------------------------------- ## 2.6. Is it reasonable to propose incompatible changes to Python? == 是否可以提出對 Python 做不相容的改動 ? == ## In general, no. There are already millions of lines of Python code around the world, so any changes in the language that invalidates more than a very small fraction of existing programs has to be frowned upon. Even if you can provide a conversion program, there still is the problem of updating all documentation. Providing a gradual upgrade path is the only way if a feature has to be changed. ## See http://www.python.org/peps/pep-0005.html for the proposed mechanism for creating backwards-incompatibilities. ## Edit this entry / Log info / Last changed on Mon Apr 1 22:13:47 2002 by Fred Drake 一般來說不要這樣做. 世界各地用 Python 寫成的程式已達數以百萬行, 因此如果新的修改, 哪怕多麼少, 假如會引致目前的程式無法正常運行的話會造成很大的困擾. 就算你能夠提供轉換程式, 但始終也要修正說明文件. 如果一定要加入這樣的功能, 我們只能以很慢的速度漸漸地升級. 關於如何提出不相容的修改建議, 請到 http://www.python.org/peps/pep-0005.html ## -------------------------------------------------------------------------------- ## 2.7. What is the future of Python? == Python 的未來 ? == ## Please see http://www.python.org/peps/ for proposals of future activities. One of the PEPs (Python Enhancement Proposals) deals with the PEP process and PEP format -- see http://www.python.org/peps/pep-0001.html if you want to submit a PEP. In http://www.python.org/peps/pep-0042.html there is a list of wishlists the Python Development team plans to tackle. ## Edit this entry / Log info / Last changed on Mon Apr 1 22:15:46 2002 by Fred Drake 請到 http://www.python.org/peps/ 閱讀有關未來的活動. 其中一章是關於提交 PEP 的規定和其格式 http://www.python.org/peps/pep-0001.html. 另外 http://www.python.org/peps/pep-0042.html 中有一張表羅列了開發人員們打算完成的計劃. ## -------------------------------------------------------------------------------- ## 2.8. What was the PSA, anyway? == 到底 PSA 是甚麼 ? == ## The Python Software Activity was created by a number of Python aficionados who want Python to be more than the product and responsibility of a single individual. The PSA was not an independent organization, but lived under the umbrealla of CNRI. ## The PSA has been superseded by the Python Software Foundation, an independent non-profit organization. The PSF's home page is at http://www.python.org/psf/. ## Some pages created by the PSA still live at http://www.python.org/psa/ ## Edit this entry / Log info / Last changed on Thu Jul 25 18:19:44 2002 by GvR PSA 全名是 Python Software Activity 是由多名 Python 核心成員組成, 其成立目地是為了避免 Python 成為單獨個人的產品並由一個人全權負責. PSA 本身並不是一個獨立的組織, 它是屬於 CNRI 的. 後來 Python Software Foundation (PSF, Python 軟體基金) 成立並取締了 PSA. PSF 為一不牟利團體, 其網頁請到 http://www.python.org/psf/ 在 http://www.python.org./psa/ 上還有部分 PSA 時期的文件. ## -------------------------------------------------------------------------------- ## 2.9. Deleted == 經已刪除 == ## Edit this entry / Log info / Last changed on Tue Jan 2 02:51:30 2001 by Moshe Zadka ## -------------------------------------------------------------------------------- ## 2.10. Deleted == 經已刪除 == ## Edit this entry / Log info / Last changed on Tue Jan 2 02:52:19 2001 by Moshe Zadka ## -------------------------------------------------------------------------------- ## 2.11. Is Python Y2K (Year 2000) Compliant? == Python 是否沒有公元二千的數位問題 ? == ## As of January, 2001 no major problems have been reported and Y2K compliance seems to be a non-issue. ## Since Python is available free of charge, there are no absolute guarantees. If there are unforeseen problems, liability is the user's rather than the developers', and there is nobody you can sue for damages. ## Python does few date manipulations, and what it does is all based on the Unix representation for time (even on non-Unix systems) which uses seconds since 1970 and won't overflow until 2038. ## Edit this entry / Log info / Last changed on Mon Jan 8 17:19:32 2001 by Steve Holden 到 2001 年 1 月止, 並未收到式發現任何有關公元二千的重大問題, 相信 Python 語言並不存在這個問題. 然而由於 Python 是免費軟件, 我們不能絕對保證它的安全程度. 如果有潛在的問題發生了, Python 開發者並不會對此負責, 用戶並不能據此向開發人員索償. Python 有一些處理日期和時間的函數, 這些統統都建基於 Unix 機上的時間表示方法 (就算在非 Unix 機種上也用這種方法), 時間是用自 1970 年起過了多少秒來算的, 這個計量方法在 2038 年以前都不會出現溢值錯誤. ## -------------------------------------------------------------------------------- ## 2.12. Is Python a good language in a class for beginning programmers? == Python 是不是利於編程入門者初學的語言? == (譯按: 絕對沒錯. 理由很多, 慢慢再告訴你.) ## Yes. This long answer attempts to address any concerns you might have with teaching Python as a programmer's first language. (If you want to discuss Python's use in education, then you may be interested in joining the edu-sig mailinglist. See http://www.python.org/sigs/edu-sig/ ) ## It is still common to start students with a procedural (subset of a) statically typed language such as Pascal, C, or a subset of C++ or Java. I think that students may be better served by learning Python as their first language. Python has a very simple and consistent syntax and a large standard library. Most importantly, using Python in a beginning programming course permits students to concentrate on important programming skills, such as problem decomposition and data type design. ## With Python, students can be quickly introduced to basic concepts such as loops and procedures. They can even probably work with user-defined objects in their very first course. They could implement a tree structure as nested Python lists, for example. They could be introduced to objects in their first course if desired. For a student who has never programmed before, using a statically typed language seems unnatural. It presents additional complexity that the student must master and slows the pace of the course. The students are trying to learn to think like a computer, decompose problems, design consistent interfaces, and encapsulate data. While learning to use a statically typed language is important, it is not necessarily the best topic to address in the students' first programming course. ## Many other aspects of Python make it a good first language. Python has a large standard library (like Java) so that students can be assigned programming projects very early in the course that do something. Assignments aren't restricted to the standard four-function calculator and check balancing programs. By using the standard library, students can gain the satisfaction of working on realistic applications as they learn the fundamentals of programming. Using the standard library also teaches students about code reuse. ## Python's interactive interpreter also enables students to test language features while they're programming. They can keep a window with the interpreter running while they enter their programs' source in another window. If they can't remember the methods for a list, they can do something like this: ## >>> L = [] ## >>> dir(L) ## ['append', 'count', 'extend', 'index', 'insert', 'pop', 'remove', ## 'reverse', 'sort'] ## >>> print L.append.__doc__ ## L.append(object) -- append object to end ## >>> L.append(1) ## >>> L ## [1] ## With the interpreter, documentation is never far from the student as he's programming. ## There are also good IDEs for Python. Guido van Rossum's IDLE is a cross-platform IDE for Python that is written in Python using Tk. There is also a Windows specific IDE called PythonWin. Emacs users will be happy to know that there is a very good Python mode for Emacs. All of these programming environments provide syntax highlighting, auto-indenting, and access to the interactive interpreter while coding. For more information about IDEs, see XXX. ## If your department is currently using Pascal because it was designed to be a teaching language, then you'll be happy to know that Guido van Rossum designed Python to be simple to teach to everyone but powerful enough to implement real world applications. Python makes a good language for first time programmers because that was one of Python's design goals. There are papers at http://www.python.org/doc/essays/ on the Python website by Python's creator explaining his objectives for the language. One that may interest you is titled "Computer Programming for Everybody" http://www.python.org/doc/essays/cp4e.html ## If you're seriously considering Python as a language for you school, Guido van Rossum may even be willing to correspond with you about how the language would fit in your curriculum. See http://www.python.org/doc/FAQ.html#2.2 for examples of Python's use in the "real world." ## While Python, its source code, and its IDEs are freely available, this consideration should not rule out other languages. There are other free languages (Java, free C compilers), and many companies are willing to waive some or all of their fees for student programming tools if it guarantees that a whole graduating class will know how to use their tools. That is, if one of the requirements for the language that will be taught is that it be freely available, then Python qualifies, but this requirement does not preclude other languages. ## While Python jobs may not be as prevalent as C/C++/Java jobs, teachers should not worry about teaching students critical job skills in their first course. The skills that win students a job are those they learn in their senior classes and internships. Their first programming courses are there to lay a solid foundation in programming fundamentals. The primary question in choosing the language for such a course should be which language permits the students to learn this material without hindering or limiting them. ## Another argument for Python is that there are many tasks for which something like C++ is overkill. That's where languages like Python, Perl, Tcl, and Visual Basic thrive. It's critical for students to know something about these languages. (Every employer for whom I've worked used at least one such language.) Of the languages listed above, Python probably makes the best language in a programming curriculum since its syntax is simple, consistent, and not unlike other languages (C/C++/Java) that are probably in the curriculum. By starting students with Python, a department simultaneously lays the foundations for other programming courses and introduces students to the type of language that is often used as a "glue" language. As an added bonus, Python can be used to interface with Microsoft's COM components (thanks to Mark Hammond). There is also Jython, a Java implementation of the Python interpreter, that can be used to connect Java components. ## If you currently start students with Pascal or C/C++ or Java, you may be worried they will have trouble learning a statically typed language after starting with Python. I think that this fear most often stems from the fact that the teacher started with a statically typed language, and we tend to like to teach others in the same way we were taught. In reality, the transition from Python to one of these other languages is quite simple. ## To motivate a statically typed language such as C++, begin the course by explaining that unlike Python, their first language, C++ is compiled to a machine dependent executable. Explain that the point is to make a very fast executable. To permit the compiler to make optimizations, programmers must help it by specifying the "types" of variables. By restricting each variable to a specific type, the compiler can reduce the book-keeping it has to do to permit dynamic types. The compiler also has to resolve references at compile time. Thus, the language gains speed by sacrificing some of Python's dynamic features. Then again, the C++ compiler provides type safety and catches many bugs at compile time instead of run time (a critical consideration for many commercial applications). C++ is also designed for very large programs where one may want to guarantee that others don't touch an object's implementation. C++ provides very strong language features to separate an object's implementation from its interface. Explain why this separation is a good thing. ## The first day of a C++ course could then be a whirlwind introduction to what C++ requires and provides. The point here is that after a semester or two of Python, students are hopefully competent programmers. They know how to handle loops and write procedures. They've also worked with objects, thought about the benefits of consistent interfaces, and used the technique of subclassing to specialize behavior. Thus, a whirlwind introduction to C++ could show them how objects and subclassing looks in C++. The potentially difficult concepts of object-oriented design were taught without the additional obstacles presented by a language such as C++ or Java. When learning one of these languages, the students would already understand the "road map." They understand objects; they would just be learning how objects fit in a statically typed languages. Language requirements and compiler errors that seem unnatural to beginning programmers make sense in this new context. Many students will find it helpful to be able to write a fast prototype of their algorithms in Python. Thus, they can test and debug their ideas before they attempt to write the code in the new language, saving the effort of working with C++ types for when they've discovered a working solution for their assignments. When they get annoyed with the rigidity of types, they'll be happy to learn about containers and templates to regain some of the lost flexibility Python afforded them. Students may also gain an appreciation for the fact that no language is best for every task. They'll see that C++ is faster, but they'll know that they can gain flexibility and development speed with a Python when execution speed isn't critical. ## If you have any concerns that weren't addressed here, try posting to the Python newsgroup. Others there have done some work with using Python as an instructional tool. Good luck. We'd love to hear about it if you choose Python for your course. ## Edit this entry / Log info / Last changed on Mon Apr 15 02:55:02 2002 by michael favor ## -------------------------------------------------------------------------------- ## 3. Building Python and Other Known Bugs = 編譯時的問題以及其他各項已知的錯誤 = ## -------------------------------------------------------------------------------- ## 3.1. Is there a test set? == 有沒有一套測試程式可以知道我的案裝是否完滿 ? == ## Sure. You can run it after building with "make test", or you can run it manually with this command at the Python prompt: ## import test.autotest ## In Python 1.4 or earlier, use ## import autotest ## The test set doesn't test all features of Python, but it goes a long way to confirm that Python is actually working. ## NOTE: if "make test" fails, don't just mail the output to the newsgroup -- this doesn't give enough information to debug the problem. Instead, find out which test fails, and run that test manually from an interactive interpreter. For example, if "make test" reports that test_spam fails, try this interactively: ## import test.test_spam ## This generally produces more verbose output which can be diagnosed to debug the problem. If you find a bug in Python or the libraries, or in the tests, please report this in the Python bug tracker at SourceForge: ## http://sourceforge.net/tracker/?func=add&group_id=5470&atid=105470 ## Edit this entry / Log info / Last changed on Fri Apr 27 10:29:36 2001 by Fred Drake 有的. 你可以在編譯完成後鍵入 "make test" 來運行測試套件. 或者, 你可以在 Python 中輸入: ＞＞＞import test.autotest 測試套件不會檢驗所有的功能, 但它會試運行許許多多的指令以確認 Python 是正常的. ## -------------------------------------------------------------------------------- ## 3.2. When running the test set, I get complaints about floating point operations, but when playing with floating point operations I cannot find anything wrong with them. == 運行測試程式時, 它回報說遇到浮點運算的錯誤 == ''', 但我真的用 Python 寫程做浮點運算時並不曾發現錯誤, 為什麼?''' ## The test set makes occasional unwarranted assumptions about the semantics of C floating point operations. Until someone donates a better floating point test set, you will have to comment out the offending floating point tests and execute similar tests manually. ## Edit this entry / Log info 這是因為寫這部分測試程式的人對 C 語言上的浮點運算的預期值做了一些不當的假設 (也許並不能說是不當, 只是在某些情況下這類假設並不保證絕對符合). 因此除非有人改正了這部分的測試程式, 否則你要自行把它注釋掉. 你也可能需要手動運行別的測試集. ## -------------------------------------------------------------------------------- ## 3.3. Link errors after rerunning the configure script. == 執行好 configure 之後在 Link 步骤發生錯誤. == ## It is generally necessary to run "make clean" after a configuration change. ## Edit this entry / Log info 普遍來說, 系統設定改變之後 (就是你的 configure script 變了) 再編譯時要先執行 "make clean" 指令. ## -------------------------------------------------------------------------------- ## 3.4. The python interpreter complains about options passed to a script (after the script name). == Python 不接受傳給程式的參數 == ## You are probably linking with GNU getopt, e.g. through -liberty. Don't. The reason for the complaint is that GNU getopt, unlike System V getopt and other getopt implementations, doesn't consider a non-option to be the end of the option list. A quick (and compatible) fix for scripts is to add "--" to the interpreter, like this: ## #! /usr/local/bin/python -- ## You can also use this interactively: ## python -- script.py [options] ## Note that a working getopt implementation is provided in the Python distribution (in Python/getopt.c) but not automatically used. ## Edit this entry / Log info 你大概是用了 GNU 的 getopt, 例如用 -liberty 來編譯, 別這樣做. 理由是 GNU 的 getopt 和 System V 的不同, 它遇到非參數項時不把它當作是參數列的終結. 最快的修正方法(相容的)是在程式前加上 '--' 號, 像這樣: －　#! /usr/local/bin/python -- 如果是在互動態下： －　python -- script.py [options] 另外在 Python 的源碼中其實有一個修改過的 getopt 版本(位於 Python/getopt.c), 不過正常編譯態下不會使用它. (譯按: 應可在運行 configure 時加入相關選項.) ## -------------------------------------------------------------------------------- ## 3.5. When building on the SGI, make tries to run python to create glmodule.c, but python hasn't been built or installed yet. ## Comment out the line mentioning glmodule.c in Setup and build a python without gl first; install it or make sure it is in your $PATH, then edit the Setup file again to turn on the gl module, and make again. You don't need to do "make clean"; you do need to run "make Makefile" in the Modules subdirectory (or just run "make" at the toplevel). ## Edit this entry / Log info ## -------------------------------------------------------------------------------- ## 3.6. I use VPATH but some targets are built in the source directory. ## On some systems (e.g. Sun), if the target already exists in the source directory, it is created there instead of in the build directory. This is usually because you have previously built without VPATH. Try running "make clobber" in the source directory. ## Edit this entry / Log info ## -------------------------------------------------------------------------------- ## 3.7. Trouble building or linking with the GNU readline library. ## You can use the GNU readline library to improve the interactive user interface: this gives you line editing and command history when calling python interactively. Its sources are distributed with Python (at least for 2.0). Uncomment the line ## #readline readline.c -lreadline -ltermcap ## in Modules/Setup. The configuration option --with-readline is no longer supported, at least in Python 2.0. Some hints on building and using the readline library: On SGI IRIX 5, you may have to add the following to rldefs.h: ## #ifndef sigmask ## #define sigmask(sig) (1L << ((sig)-1)) ## #endif ## On some systems, you will have to add #include "rldefs.h" to the top of several source files, and if you use the VPATH feature, you will have to add dependencies of the form foo.o: foo.c to the Makefile for several values of foo. The readline library requires use of the termcap library. A known problem with this is that it contains entry points which cause conflicts with the STDWIN and SGI GL libraries. The STDWIN conflict can be solved by adding a line saying '#define werase w_erase' to the stdwin.h file (in the STDWIN distribution, subdirectory H). The GL conflict has been solved in the Python configure script by a hack that forces use of the static version of the termcap library. Check the newsgroup gnu.bash.bug news:gnu.bash.bug for specific problems with the readline library (I don't read this group but I've been told that it is the place for readline bugs). ## Edit this entry / Log info / Last changed on Sat Dec 2 18:23:48 2000 by Issac Trotts ## -------------------------------------------------------------------------------- ## 3.8. Trouble with socket I/O on older Linux 1.x versions. ## Once you've built Python, use it to run the regen script in the Lib/plat-linux2 directory. Apparently the files as distributed don't match the system headers on some Linux versions. ## Note that this FAQ entry only applies to Linux kernel versions 1.x.y; these are hardly around any more. ## Edit this entry / Log info / Last changed on Tue Jul 30 20:05:52 2002 by Jens Kubieziel ## -------------------------------------------------------------------------------- ## 3.9. Trouble with prototypes on Ultrix. ## Ultrix cc seems broken -- use gcc, or edit config.h to #undef HAVE_PROTOTYPES. ## Edit this entry / Log info ## -------------------------------------------------------------------------------- ## 3.10. Other trouble building Python on platform X. ## Please submit the details to the SourceForge bug tracker: ## http://sourceforge.net/tracker/?group_id=5470&atid=105470 ## and we'll look into it. Please provide as many details as possible. In particular, if you don't tell us what type of computer and what operating system (and version) you are using it will be difficult for us to figure out what is the matter. If you have compilation output logs, please use file uploads -- don't paste everything in the message box. ## In many cases, we won't have access to the same hardware or operating system version, so please, if you have a SourceForge account, log in before filing your report, or if you don't have an account, include an email address at which we can reach you for further questions. Logging in to SourceForge first will also cause SourceForge to send you updates as we act on your report. ## Edit this entry / Log info / Last changed on Fri Apr 27 10:53:18 2001 by Fred Drake ## -------------------------------------------------------------------------------- ## 3.11. How to configure dynamic loading on Linux. ## This is now automatic as long as your Linux version uses the ELF object format (all recent Linuxes do). ## Edit this entry / Log info ## -------------------------------------------------------------------------------- ## 3.12. I can't get shared modules to work on Linux 2.0 (Slackware96)? ## This is a bug in the Slackware96 release. The fix is simple: Make sure that there is a link from /lib/libdl.so to /lib/libdl.so.1 so that the following links are setup: /lib/libdl.so -> /lib/libdl.so.1 /lib/libdl.so.1 -> /lib/libdl.so.1.7.14 You may have to rerun the configure script, after rm'ing the config.cache file, before you attempt to rebuild python after this fix. ## Edit this entry / Log info / Last changed on Wed May 21 15:45:03 1997 by GvR ## -------------------------------------------------------------------------------- ## 3.13. Trouble when making modules shared on Linux. ## This happens when you have built Python for static linking and then enable ## *shared* ## in the Setup file. Shared library code must be compiled with "-fpic". If a .o file for the module already exist that was compiled for static linking, you must remove it or do "make clean" in the Modules directory. ## Edit this entry / Log info / Last changed on Fri May 23 13:42:30 1997 by GvR ## -------------------------------------------------------------------------------- ## 3.14. [deleted] ## [ancient information on threads on linux (when thread support was not standard) used to be here] ## Edit this entry / Log info / Last changed on Sun Jun 2 17:27:13 2002 by Erno Kuusela ## -------------------------------------------------------------------------------- ## 3.15. Errors when linking with a shared library containing C++ code. ## Link the main Python binary with C++. Change the definition of LINKCC in Modules/Makefile to be your C++ compiler. You may have to edit config.c slightly to make it compilable with C++. ## Edit this entry / Log info ## -------------------------------------------------------------------------------- ## 3.16. Deleted ## Edit this entry / Log info / Last changed on Tue Sep 11 16:02:22 2001 by GvR ## -------------------------------------------------------------------------------- ## 3.17. Deleted. ## Edit this entry / Log info / Last changed on Tue Sep 11 15:54:57 2001 by GvR ## -------------------------------------------------------------------------------- ## 3.18. Compilation or link errors for the _tkinter module ## Most likely, there's a version mismatch between the Tcl/Tk header files (tcl.h and tk.h) and the Tcl/Tk libraries you are using e.g. "-ltk8.0" and "-ltcl8.0" arguments for _tkinter in the Setup file). It is possible to install several versions of the Tcl/Tk libraries, but there can only be one version of the tcl.h and tk.h header files. If the library doesn't match the header, you'll get problems, either when linking the module, or when importing it. Fortunately, the version number is clearly stated in each file, so this is easy to find. Reinstalling and using the latest version usually fixes the problem. ## (Also note that when compiling unpatched Python 1.5.1 against Tcl/Tk 7.6/4.2 or older, you get an error on Tcl_Finalize. See the 1.5.1 patch page at http://www.python.org/1.5/patches-1.5.1/.) ## Edit this entry / Log info / Last changed on Thu Jun 11 00:49:14 1998 by Gvr ## -------------------------------------------------------------------------------- ## 3.19. I configured and built Python for Tcl/Tk but "import Tkinter" fails. ## Most likely, you forgot to enable the line in Setup that says "TKPATH=:$(DESTLIB)/tkinter". ## Edit this entry / Log info ## -------------------------------------------------------------------------------- ## 3.20. [deleted] ## [ancient information on a gcc+tkinter bug on alpha was here] ## Edit this entry / Log info / Last changed on Mon Jun 3 16:46:23 2002 by Erno Kuusela ## -------------------------------------------------------------------------------- ## 3.21. Several common system calls are missing from the posix module. ## Most likely, all test compilations run by the configure script are failing for some reason or another. Have a look in config.log to see what could be the reason. A common reason is specifying a directory to the --with-readline option that doesn't contain the libreadline.a file. ## Edit this entry / Log info ## -------------------------------------------------------------------------------- ## 3.22. ImportError: No module named string, on MS Windows. ## Most likely, your PYTHONPATH environment variable should be set to something like: ## set PYTHONPATH=c:\python;c:\python\lib;c:\python\scripts ## (assuming Python was installed in c:\python) ## Edit this entry / Log info ## -------------------------------------------------------------------------------- ## 3.23. Core dump on SGI when using the gl module. ## There are conflicts between entry points in the termcap and curses libraries and an entry point in the GL library. There's a hack of a fix for the termcap library if it's needed for the GNU readline library, but it doesn't work when you're using curses. Concluding, you can't build a Python binary containing both the curses and gl modules. ## Edit this entry / Log info ## -------------------------------------------------------------------------------- ## 3.24. "Initializer not a constant" while building DLL on MS-Windows ## Static type object initializers in extension modules may cause compiles to fail with an error message like "initializer not a constant". Fredrik Lundh explains: ## This shows up when building DLL under MSVC. There's two ways to address this: either compile the module as C++, or change your code to something like: ## statichere PyTypeObject bstreamtype = { ## PyObject_HEAD_INIT(NULL) /* must be set by init function */ ## 0, ## "bstream", ## sizeof(bstreamobject), ## ... ## void ## initbstream() ## { ## /* Patch object type */ ## bstreamtype.ob_type = &PyType_Type; ## Py_InitModule("bstream", functions); ## ... ## } ## Edit this entry / Log info / Last changed on Sun May 25 14:58:05 1997 by Aaron Watters ## -------------------------------------------------------------------------------- ## 3.25. Output directed to a pipe or file disappears on Linux. ## Some people have reported that when they run their script interactively, it runs great, but that when they redirect it to a pipe or file, no output appears. ## % python script.py ## ...some output... ## % python script.py >file ## % cat file ## % # no output ## % python script.py | cat ## % # no output ## % ## Nobody knows what causes this, but it is apparently a Linux bug. Most Linux users are not affected by this. ## There's at least one report of someone who reinstalled Linux (presumably a newer version) and Python and got rid of the problem; so this may be the solution. ## Edit this entry / Log info / Last changed on Mon May 26 11:04:56 1997 by GvR ## -------------------------------------------------------------------------------- ## 3.26. [deleted] ## [ancient libc/linux problem was here] ## Edit this entry / Log info / Last changed on Mon Jun 3 16:48:08 2002 by Erno Kuusela ## -------------------------------------------------------------------------------- ## 3.27. [deleted] ## [ancient linux + threads + tk problem was described here] ## Edit this entry / Log info / Last changed on Mon Jun 3 16:49:08 2002 by Erno Kuusela ## -------------------------------------------------------------------------------- ## 3.28. How can I test if Tkinter is working? ## Try the following: ## python ## >>> import _tkinter ## >>> import Tkinter ## >>> Tkinter._test() ## This should pop up a window with two buttons, one "Click me" and one "Quit". ## If the first statement (import _tkinter) fails, your Python installation probably has not been configured to support Tcl/Tk. On Unix, if you have installed Tcl/Tk, you have to rebuild Python after editing the Modules/Setup file to enable the _tkinter module and the TKPATH environment variable. ## It is also possible to get complaints about Tcl/Tk version number mismatches or missing TCL_LIBRARY or TK_LIBRARY environment variables. These have to do with Tcl/Tk installation problems. ## A common problem is to have installed versions of tcl.h and tk.h that don't match the installed version of the Tcl/Tk libraries; this usually results in linker errors or (when using dynamic loading) complaints about missing symbols during loading the shared library. ## Edit this entry / Log info / Last changed on Thu Aug 28 17:01:46 1997 by Guido van Rossum ## -------------------------------------------------------------------------------- ## 3.29. Is there a way to get the interactive mode of the python interpreter to perform function/variable name completion? ## (From a posting by Guido van Rossum) ## On Unix, if you have enabled the readline module (i.e. if Emacs-style command line editing and bash-style history works for you), you can add this by importing the undocumented standard library module "rlcompleter". When completing a simple identifier, it completes keywords, built-ins and globals in __main__; when completing NAME.NAME..., it evaluates (!) the expression up to the last dot and completes its attributes. ## This way, you can do "import string", type "string.", hit the completion key twice, and see the list of names defined by the string module. ## Tip: to use the tab key as the completion key, call ## readline.parse_and_bind("tab: complete") ## You can put this in a ~/.pythonrc file, and set the PYTHONSTARTUP environment variable to ~/.pythonrc. This will cause the completion to be enabled whenever you run Python interactively. ## Notes (see the docstring for rlcompleter.py for more information): ## * The evaluation of the NAME.NAME... form may cause arbitrary application defined code to be executed if an object with a __getattr__ hook is found. Since it is the responsibility of the application (or the user) to enable this feature, I consider this an acceptable risk. More complicated expressions (e.g. function calls or indexing operations) are not evaluated. ## * GNU readline is also used by the built-in functions input() and raw_input(), and thus these also benefit/suffer from the complete features. Clearly an interactive application can benefit by specifying its own completer function and using raw_input() for all its input. ## * When stdin is not a tty device, GNU readline is never used, and this module (and the readline module) are silently inactive. ## Edit this entry / Log info / Last changed on Fri Jun 12 09:55:24 1998 by A.M. Kuchling ## -------------------------------------------------------------------------------- ## 3.30. Why is the Python interpreter not built as a shared library? ## (This is a Unix question; on Mac and Windows, it is a shared library.) ## It's just a nightmare to get this to work on all different platforms. Shared library portability is a pain. And yes, I know about GNU libtool -- but it requires me to use its conventions for filenames etc, and it would require a complete and utter rewrite of all the makefile and config tools I'm currently using. ## In practice, few applications embed Python -- it's much more common to have Python extensions, which already are shared libraries. Also, serious embedders often want total control over which Python version and configuration they use so they wouldn't want to use a standard shared library anyway. So while the motivation of saving space when lots of apps embed Python is nice in theory, I doubt that it will save much in practice. (Hence the low priority I give to making a shared library.) ## For Linux systems, the simplest method of producing libpython1.5.so seems to be (originally from the Minotaur project web page, http://www.equi4.com/minotaur/minotaur.html): ## make distclean ## ./configure ## make OPT="-fpic -O2" ## mkdir .extract ## (cd .extract; ar xv ../libpython1.5.a) ## gcc -shared -o libpython1.5.so .extract/*.o ## rm -rf .extract ## In Python 2.3 this will be supported by the standard build routine (at least on Linux) with --enable-shared. Note however that there is little advantage, and it slows down Python because of the need for PIC code and the extra cost at startup time to find the library. ## Edit this entry / Log info / Last changed on Thu May 30 13:36:55 2002 by GvR ## -------------------------------------------------------------------------------- ## 3.31. Build with GCC on Solaris 2.6 (SunOS 5.6) fails ## If you have upgraded Solaris 2.5 or 2.5.1 to Solaris 2.6, but you have not upgraded your GCC installation, the compile may fail, e.g. like this: ## In file included from /usr/include/sys/stream.h:26, ## from /usr/include/netinet/in.h:38, ## from /usr/include/netdb.h:96, ## from ./socketmodule.c:121: ## /usr/include/sys/model.h:32: #error "No DATAMODEL_NATIVE specified" ## Solution: rebuild GCC for Solaris 2.6. You might be able to simply re-run fixincludes, but people have had mixed success with doing that. ## Edit this entry / Log info / Last changed on Wed Oct 21 11:18:46 1998 by GvR ## -------------------------------------------------------------------------------- ## 3.32. Running "make clean" seems to leave problematic files that cause subsequent builds to fail. ## Use "make clobber" instead. ## Use "make clean" to reduce the size of the source/build directory after you're happy with your build and installation. If you have already tried to build python and you'd like to start over, you should use "make clobber". It does a "make clean" and also removes files such as the partially built Python library from a previous build. ## Edit this entry / Log info / Last changed on Thu Jun 24 20:39:26 1999 by TAB ## -------------------------------------------------------------------------------- ## 3.33. Submitting bug reports and patches ## To report a bug or submit a patch, please use the relevant service from the Python project at SourceForge. ## Bugs: http://sourceforge.net/tracker/?group_id=5470&atid=105470 ## Patches: http://sourceforge.net/tracker/?group_id=5470&atid=305470 ## If you have a SourceForge account, please log in before submitting your bug report; this will make it easier for us to contact you regarding your report in the event we have follow-up questions. It will also enable SourceForge to send you update information as we act on your bug. If you do not have a SourceForge account, please consider leaving your name and email address as part of the report. ## Edit this entry / Log info / Last changed on Fri Apr 27 10:58:26 2001 by Fred Drake ## -------------------------------------------------------------------------------- ## 3.34. I can't load shared libraries under Python 1.5.2, Solaris 7, and gcc 2.95.2 ## When trying to load shared libraries, you may see errors like: ImportError: ld.so.1: python: fatal: relocation error: file /usr/local/lib/python1.5/site-packages/Perp/util/du_SweepUtilc.so: ## symbol PyExc_RuntimeError: referenced symbol not found ## There is a problem with the configure script for Python 1.5.2 under Solaris 7 with gcc 2.95 . configure should set the make variable LINKFORSHARED=-Xlinker -export-dynamic ## in Modules/Makefile, ## Manually add this line to the Modules/Makefile. This builds a Python executable that can load shared library extensions (xxx.so) . ## Edit this entry / Log info / Last changed on Mon Feb 19 10:37:05 2001 by GvR ## -------------------------------------------------------------------------------- ## 3.35. In the regression test, test___all__ fails for the profile module. What's wrong? ## If you have been using the profile module, and have properly calibrated a copy of the module as described in the documentation for the profiler: ## http://www.python.org/doc/current/lib/profile-calibration.html ## then it is possible that the regression test "test___all__" will fail if you run the regression test manually rather than using "make test" in the Python source directory. This will happen if you have set your PYTHONPATH environment variable to include the directory containing your calibrated profile module. You have probably calibrated the profiler using an older version of the profile module which does not define the __all__ value, added to the module as of Python 2.1. ## The problem can be fixed by removing the old calibrated version of the profile module and using the latest version to do a fresh calibration. In general, you will need to re-calibrate for each version of Python anyway, since the performance characteristics can change in subtle ways that impact profiling. ## Edit this entry / Log info / Last changed on Fri Apr 27 10:44:10 2001 by Fred Drake ## -------------------------------------------------------------------------------- ## 3.36. relocations remain against allocatable but non-writable sections ## This linker error occurs on Solaris if you attempt to build an extension module which incorporates position-dependent (non-PIC) code. A common source of problems is that a static library (.a file), such as libreadline.a or libcrypto.a is linked with the extension module. The error specifically occurs when using gcc as the compiler, but /usr/ccs/bin/ld as the linker. ## The following solutions and work-arounds are known: ## 1. Rebuild the libraries (libreadline, libcrypto) with -fPIC (-KPIC if using the system compiler). This is recommended; all object files in a shared library should be position-independent. ## 2. Statically link the extension module and its libraries into the Python interpreter, by editing Modules/Setup. ## 3. Use GNU ld instead of /usr/ccs/bin/ld; GNU ld will accept non-PIC code in shared libraries (and mark the section writable) ## 4. Pass -mimpure-text to GCC when linking the module. This will force gcc to not pass -z text to ld; in turn, ld will make all text sections writable. ## Options 3 and 4 are not recommended, since the ability to share code across processes is lost. ## -------------------------------------------------------------------------------- ## 4. Programming in Python = Python 編程 = ## -------------------------------------------------------------------------------- ## 4.1. Is there a source code level debugger with breakpoints, step, etc.? ## Yes. Module pdb, documented in the Library Reference Manual is a rudimentary but adequate debugger for Python. You can also write your own debugger by using the code for pdb as an example. ## Pythonwin also has a GUI debugger available, based on bdb, which colors breakpoints and has quite a few cool features (including debugging non-Pythonwin programs). The interface needs some work, but is interesting none the less. A reference can be found in ## http://www.python.org/ftp/python/pythonwin/pwindex.html ## More recent version of PythonWin are available as a part of ActivePython. See ## http://www.activestate.com/Products/ActivePython/index.html ## Richard Wolff has created a modified version of pdb, called Pydb, for use with the popular Data Display Debugger (DDD). Pydb can be found at http://daikon.tuc.noao.edu/python/, and DDD can be found at http://www.cs.tu-bs.de/softech/ddd/ ## The IDLE interactive development environment, normally available at Tools/idle in a standard distribution, also contains a graphical debugger. ## Edit this entry / Log info / Last changed on Mon Jan 28 16:27:01 2002 by Victor Kryukov ## -------------------------------------------------------------------------------- ## 4.2. Can I create an object class with some methods implemented in C and others in Python (e.g. through inheritance)? (Also phrased as: Can I use a built-in type as base class?) ## In Python 2.2, you can inherit from builtin classes such as int, list, dict, etc. ## In previous versions of Python, you can easily create a Python class which serves as a wrapper around a built-in object, e.g. (for dictionaries): ## # A user-defined class behaving almost identical ## # to a built-in dictionary. ## class UserDict: ## def __init__(self): self.data = {} ## def __repr__(self): return repr(self.data) ## def __cmp__(self, dict): ## if type(dict) == type(self.data): ## return cmp(self.data, dict) ## else: ## return cmp(self.data, dict.data) ## def __len__(self): return len(self.data) ## def __getitem__(self, key): return self.data[key] ## def __setitem__(self, key, item): self.data[key] = item ## def __delitem__(self, key): del self.data[key] ## def keys(self): return self.data.keys() ## def items(self): return self.data.items() ## def values(self): return self.data.values() ## def has_key(self, key): return self.data.has_key(key) ## A2. See Jim Fulton's ExtensionClass for an example of a mechanism which allows you to have superclasses which you can inherit from in Python -- that way you can have some methods from a C superclass (call it a mixin) and some methods from either a Python superclass or your subclass. ExtensionClass is distributed as a part of Zope (see http://www.zope.org), but will be phased out with Zope 3, since Zope 3 uses Python 2.2 or later which supports direct inheritance from built-in types. Here's a link to the original paper about ExtensionClass: http://debian.acm.ndsu.nodak.edu/doc/python-extclass/ExtensionClass.html ## A3. The Boost Python Library (BPL, http://www.boost.org/libs/python/doc/index.html) provides a way of doing this from C++ (i.e. you can inherit from an extension class written in C++ using the BPL). ## Edit this entry / Log info / Last changed on Tue May 28 21:09:52 2002 by GvR ## -------------------------------------------------------------------------------- ## 4.3. Is there a curses/termcap package for Python? ## The standard Python source distribution comes with a curses module in the Modules/ subdirectory, though it's not compiled by default (note that this is not available in the Windows distribution -- there is no curses module for Windows). ## In Python versions before 2.0 the module only supported plain curses; you couldn't use ncurses features like colors with it (though it would link with ncurses). ## In Python 2.0, the curses module has been greatly extended, starting from Oliver Andrich's enhanced version, to provide many additional functions from ncurses and SYSV curses, such as colour, alternative character set support, pads, and mouse support. This means the module is no longer compatible with operating systems that only have BSD curses, but there don't seem to be any currently maintained OSes that fall into this category. ## Edit this entry / Log info / Last changed on Sun Jun 23 20:24:06 2002 by Tim Peters ## -------------------------------------------------------------------------------- ## 4.4. Is there an equivalent to C's onexit() in Python? ## For Python 2.0: The new atexit module provides a register function that is similar to C's onexit. See the Library Reference for details. For 2.0 you should not assign to sys.exitfunc! ## For Python 1.5.2: You need to import sys and assign a function to sys.exitfunc, it will be called when your program exits, is killed by an unhandled exception, or (on UNIX) receives a SIGHUP or SIGTERM signal. ## Edit this entry / Log info / Last changed on Thu Dec 28 12:14:55 2000 by Bjorn Pettersen ## -------------------------------------------------------------------------------- ## 4.5. [deleted] ## [python used to lack nested scopes, it was explained here] ## Edit this entry / Log info / Last changed on Thu Mar 21 05:18:22 2002 by Erno Kuusela ## -------------------------------------------------------------------------------- ## 4.6. How do I iterate over a sequence in reverse order? ## If it is a list, the fastest solution is ## list.reverse() ## try: ## for x in list: ## "do something with x" ## finally: ## list.reverse() ## This has the disadvantage that while you are in the loop, the list is temporarily reversed. If you don't like this, you can make a copy. This appears expensive but is actually faster than other solutions: ## rev = list[:] ## rev.reverse() ## for x in rev: ## ## If it's not a list, a more general but slower solution is: ## for i in range(len(sequence)-1, -1, -1): ## x = sequence[i] ## ## A more elegant solution, is to define a class which acts as a sequence and yields the elements in reverse order (solution due to Steve Majewski): ## class Rev: ## def __init__(self, seq): ## self.forw = seq ## def __len__(self): ## return len(self.forw) ## def __getitem__(self, i): ## return self.forw[-(i + 1)] ## You can now simply write: ## for x in Rev(list): ## ## Unfortunately, this solution is slowest of all, due to the method call overhead... ## Edit this entry / Log info / Last changed on Sun May 25 21:10:50 1997 by GvR ## -------------------------------------------------------------------------------- ## 4.7. My program is too slow. How do I speed it up? ## That's a tough one, in general. There are many tricks to speed up Python code; I would consider rewriting parts in C only as a last resort. One thing to notice is that function and (especially) method calls are rather expensive; if you have designed a purely OO interface with lots of tiny functions that don't do much more than get or set an instance variable or call another method, you may consider using a more direct way, e.g. directly accessing instance variables. Also see the standard module "profile" (described in the Library Reference manual) which makes it possible to find out where your program is spending most of its time (if you have some patience -- the profiling itself can slow your program down by an order of magnitude). ## Remember that many standard optimization heuristics you may know from other programming experience may well apply to Python. For example it may be faster to send output to output devices using larger writes rather than smaller ones in order to avoid the overhead of kernel system calls. Thus CGI scripts that write all output in "one shot" may be notably faster than those that write lots of small pieces of output. ## Also, be sure to use "aggregate" operations where appropriate. For example the "slicing" feature allows programs to chop up lists and other sequence objects in a single tick of the interpreter mainloop using highly optimized C implementations. Thus to get the same effect as ## L2 = [] ## for i in range[3]: ## L2.append(L1[i]) ## it is much shorter and far faster to use ## L2 = list(L1[:3]) # "list" is redundant if L1 is a list. ## Note that the map() function, particularly used with builtin methods or builtin functions can be a convenient accelerator. For example to pair the elements of two lists together: ## >>> map(None, [1,2,3], [4,5,6]) ## [(1, 4), (2, 5), (3, 6)] ## or to compute a number of sines: ## >>> map( math.sin, (1,2,3,4)) ## [0.841470984808, 0.909297426826, 0.14112000806, -0.756802495308] ## The map operation completes very quickly in such cases. ## Other examples of aggregate operations include the join and split methods of string objects. For example if s1..s7 are large (10K+) strings then "".join([s1,s2,s3,s4,s5,s6,s7]) may be far faster than the more obvious s1+s2+s3+s4+s5+s6+s7, since the "summation" will compute many subexpressions, whereas join does all copying in one pass. For manipulating strings also consider the regular expression libraries and the "substitution" operations String % tuple and String % dictionary. Also be sure to use the list.sort builtin method to do sorting, and see FAQ's 4.51 and 4.59 for examples of moderately advanced usage -- list.sort beats other techniques for sorting in all but the most extreme circumstances. ## There are many other aggregate operations available in the standard libraries and in contributed libraries and extensions. ## Another common trick is to "push loops into functions or methods." For example suppose you have a program that runs slowly and you use the profiler (profile.run) to determine that a Python function ff is being called lots of times. If you notice that ff ## def ff(x): ## ...do something with x computing result... ## return result ## tends to be called in loops like (A) ## list = map(ff, oldlist) ## or (B) ## for x in sequence: ## value = ff(x) ## ...do something with value... ## then you can often eliminate function call overhead by rewriting ff to ## def ffseq(seq): ## resultseq = [] ## for x in seq: ## ...do something with x computing result... ## resultseq.append(result) ## return resultseq ## and rewrite (A) to ## list = ffseq(oldlist) ## and (B) to ## for value in ffseq(sequence): ## ...do something with value... ## Other single calls ff(x) translate to ffseq([x])[0] with little penalty. Of course this technique is not always appropriate and there are other variants, which you can figure out. ## You can gain some performance by explicitly storing the results of a function or method lookup into a local variable. A loop like ## for key in token: ## dict[key] = dict.get(key, 0) + 1 ## resolves dict.get every iteration. If the method isn't going to change, a faster implementation is ## dict_get = dict.get # look up the method once ## for key in token: ## dict[key] = dict_get(key, 0) + 1 ## Default arguments can be used to determine values once, at compile time instead of at run time. This can only be done for functions or objects which will not be changed during program execution, such as replacing ## def degree_sin(deg): ## return math.sin(deg * math.pi / 180.0) ## with ## def degree_sin(deg, factor = math.pi/180.0, sin = math.sin): ## return sin(deg * factor) ## Because this trick uses default arguments for terms which should not be changed, it should only be used when you are not concerned with presenting a possibly confusing API to your users. ## For an anecdote related to optimization, see ## http://www.python.org/doc/essays/list2str.html ## Edit this entry / Log info / Last changed on Mon Jun 3 01:03:54 2002 by Neal Norwitz ## -------------------------------------------------------------------------------- ## 4.8. When I have imported a module, then edit it, and import it again (into the same Python process), the changes don't seem to take place. What is going on? ## For reasons of efficiency as well as consistency, Python only reads the module file on the first time a module is imported. (Otherwise a program consisting of many modules, each of which imports the same basic module, would read the basic module over and over again.) To force rereading of a changed module, do this: ## import modname ## reload(modname) ## Warning: this technique is not 100% fool-proof. In particular, modules containing statements like ## from modname import some_objects ## will continue to work with the old version of the imported objects. ## Edit this entry / Log info ## -------------------------------------------------------------------------------- ## 4.9. How do I find the current module name? ## A module can find out its own module name by looking at the (predefined) global variable __name__. If this has the value '__main__' you are running as a script. ## Edit this entry / Log info ## -------------------------------------------------------------------------------- ## 4.10. I have a module in which I want to execute some extra code when it is run as a script. How do I find out whether I am running as a script? ## See the previous question. E.g. if you put the following on the last line of your module, main() is called only when your module is running as a script: ## if __name__ == '__main__': main() ## Edit this entry / Log info ## -------------------------------------------------------------------------------- ## 4.11. I try to run a program from the Demo directory but it fails with ImportError: No module named ...; what gives? ## This is probably an optional module (written in C!) which hasn't been configured on your system. This especially happens with modules like "Tkinter", "stdwin", "gl", "Xt" or "Xm". For Tkinter, STDWIN and many other modules, see Modules/Setup.in for info on how to add these modules to your Python, if it is possible at all. Sometimes you will have to ftp and build another package first (e.g. Tcl and Tk for Tkinter). Sometimes the module only works on specific platforms (e.g. gl only works on SGI machines). ## NOTE: if the complaint is about "Tkinter" (upper case T) and you have already configured module "tkinter" (lower case t), the solution is not to rename tkinter to Tkinter or vice versa. There is probably something wrong with your module search path. Check out the value of sys.path. ## For X-related modules (Xt and Xm) you will have to do more work: they are currently not part of the standard Python distribution. You will have to ftp the Extensions tar file, i.e. ftp://ftp.python.org/pub/python/src/X-extension.tar.gz and follow the instructions there. ## See also the next question. ## Edit this entry / Log info / Last changed on Mon Jul 7 00:36:18 1997 by Ken Manheimer ## -------------------------------------------------------------------------------- ## 4.12. [deleted] ## [stdwin (long dead windowing library) entry deleted] ## Edit this entry / Log info / Last changed on Thu Mar 21 08:30:13 2002 by Erno Kuusela ## -------------------------------------------------------------------------------- ## 4.13. What GUI toolkits exist for Python? ## Depending on what platform(s) you are aiming at, there are several. ## Currently supported solutions: ## Cross-platform: ## Tk: ## There's a neat object-oriented interface to the Tcl/Tk widget set, called Tkinter. It is part of the standard Python distribution and well-supported -- all you need to do is build and install Tcl/Tk and enable the _tkinter module and the TKPATH definition in Modules/Setup when building Python. This is probably the easiest to install and use, and the most complete widget set. It is also very likely that in the future the standard Python GUI API will be based on or at least look very much like the Tkinter interface. For more info about Tk, including pointers to the source, see the Tcl/Tk home page at http://www.scriptics.com. Tcl/Tk is now fully portable to the Mac and Windows platforms (NT and 95 only); you need Python 1.4beta3 or later and Tk 4.1patch1 or later. ## wxWindows: ## There's an interface to wxWindows called wxPython. wxWindows is a portable GUI class library written in C++. It supports GTK, Motif, MS-Windows and Mac as targets. Ports to other platforms are being contemplated or have already had some work done on them. wxWindows preserves the look and feel of the underlying graphics toolkit, and there is quite a rich widget set and collection of GDI classes. See the wxWindows page at http://www.wxwindows.org/ for more details. wxPython is a python extension module that wraps many of the wxWindows C++ classes, and is quickly gaining popularity amongst Python developers. You can get wxPython as part of the source or CVS distribution of wxWindows, or directly from its home page at http://alldunn.com/wxPython/. ## Gtk+: ## PyGtk bindings for the Gtk+ Toolkit by James Henstridge exist; see ftp://ftp.daa.com.au/pub/james/python/. Note that there are two incompatible bindings. If you are using Gtk+ 1.2.x you should get the 0.6.x PyGtk bindings from ## ftp://ftp.gtk.org/pub/python/v1.2 ## If you plan to use Gtk+ 2.0 with Python (highly recommended if you are just starting with Gtk), get the most recent distribution from ## ftp://ftp.gtk.org/pub/python/v2.0 ## If you are adventurous, you can also check out the source from the Gnome CVS repository. Set your CVS directory to :pserver:anonymous@anoncvs.gnome.org:/cvs/gnome and check the gnome-python module out from the repository. ## Other: ## There are also bindings available for the Qt toolkit (PyQt), and for KDE (PyKDE); see http://www.thekompany.com/projects/pykde/. ## For OpenGL bindings, see http://starship.python.net/~da/PyOpenGL. ## Platform specific: ## The Mac port has a rich and ever-growing set of modules that support the native Mac toolbox calls. See the documentation that comes with the Mac port. See ftp://ftp.python.org/pub/python/mac. Support by Jack Jansen jack@cwi.nl. ## Pythonwin by Mark Hammond (MHammond@skippinet.com.au) includes an interface to the Microsoft Foundation Classes and a Python programming environment using it that's written mostly in Python. See http://www.python.org/windows/. ## There's an object-oriented GUI based on the Microsoft Foundation Classes model called WPY, supported by Jim Ahlstrom jim@interet.com. Programs written in WPY run unchanged and with native look and feel on Windows NT/95, Windows 3.1 (using win32s), and on Unix (using Tk). Source and binaries for Windows and Linux are available in ftp://ftp.python.org/pub/python/wpy/. ## Obsolete or minority solutions: ## There's an interface to X11, including the Athena and Motif widget sets (and a few individual widgets, like Mosaic's HTML widget and SGI's GL widget) available from ftp://ftp.python.org/pub/python/src/X-extension.tar.gz. Support by Sjoerd Mullender sjoerd@cwi.nl. ## On top of the X11 interface there's the vpApp toolkit by Per Spilling, now also maintained by Sjoerd Mullender sjoerd@cwi.nl. See ftp://ftp.cwi.nl/pub/sjoerd/vpApp.tar.gz. ## For SGI IRIX only, there are unsupported interfaces to the complete GL (Graphics Library -- low level but very good 3D capabilities) as well as to FORMS (a buttons-and-sliders-etc package built on top of GL by Mark Overmars -- ftp'able from ftp://ftp.cs.ruu.nl/pub/SGI/FORMS/). This is probably also becoming obsolete, as OpenGL takes over (see above). ## There's an interface to STDWIN, a platform-independent low-level windowing interface for Mac and X11. This is totally unsupported and rapidly becoming obsolete. The STDWIN sources are at ftp://ftp.cwi.nl/pub/stdwin/. ## There is an interface to WAFE, a Tcl interface to the X11 Motif and Athena widget sets. WAFE is at http://www.wu-wien.ac.at/wafe/wafe.html. ## Edit this entry / Log info / Last changed on Mon May 13 21:40:39 2002 by Skip Montanaro ## -------------------------------------------------------------------------------- ## 4.14. Are there any interfaces to database packages in Python? ## Yes! See the Database Topic Guide at http://www.python.org/topics/database/ for details. ## Edit this entry / Log info / Last changed on Tue Jan 4 20:12:19 2000 by Barney Warplug ## -------------------------------------------------------------------------------- ## 4.15. Is it possible to write obfuscated one-liners in Python? ## Yes. See the following three examples, due to Ulf Bartelt: ## # Primes < 1000 ## print filter(None,map(lambda y:y*reduce(lambda x,y:x*y!=0, ## map(lambda x,y=y:y%x,range(2,int(pow(y,0.5)+1))),1),range(2,1000))) ## # First 10 Fibonacci numbers ## print map(lambda x,f=lambda x,f:(x<=1) or (f(x-1,f)+f(x-2,f)): f(x,f), ## range(10)) ## # Mandelbrot set ## print (lambda Ru,Ro,Iu,Io,IM,Sx,Sy:reduce(lambda x,y:x+y,map(lambda y, ## Iu=Iu,Io=Io,Ru=Ru,Ro=Ro,Sy=Sy,L=lambda yc,Iu=Iu,Io=Io,Ru=Ru,Ro=Ro,i=IM, ## Sx=Sx,Sy=Sy:reduce(lambda x,y:x+y,map(lambda x,xc=Ru,yc=yc,Ru=Ru,Ro=Ro, ## i=i,Sx=Sx,F=lambda xc,yc,x,y,k,f=lambda xc,yc,x,y,k,f:(k<=0)or (x*x+y*y ## >=4.0) or 1+f(xc,yc,x*x-y*y+xc,2.0*x*y+yc,k-1,f):f(xc,yc,x,y,k,f):chr( ## 64+F(Ru+x*(Ro-Ru)/Sx,yc,0,0,i)),range(Sx))):L(Iu+y*(Io-Iu)/Sy),range(Sy ## ))))(-2.1, 0.7, -1.2, 1.2, 30, 80, 24) ## # \___ ___/ \___ ___/ | | |__ lines on screen ## # V V | |______ columns on screen ## # | | |__________ maximum of "iterations" ## # | |_________________ range on y axis ## # |____________________________ range on x axis ## Don't try this at home, kids! ## Edit this entry / Log info / Last changed on Wed May 21 15:48:33 1997 by GvR ## -------------------------------------------------------------------------------- ## 4.16. Is there an equivalent of C's "?:" ternary operator? ## Not directly. In many cases you can mimic a?b:c with "a and b or c", but there's a flaw: if b is zero (or empty, or None -- anything that tests false) then c will be selected instead. In many cases you can prove by looking at the code that this can't happen (e.g. because b is a constant or has a type that can never be false), but in general this can be a problem. ## Tim Peters (who wishes it was Steve Majewski) suggested the following solution: (a and [b] or [c])[0]. Because [b] is a singleton list it is never false, so the wrong path is never taken; then applying [0] to the whole thing gets the b or c that you really wanted. Ugly, but it gets you there in the rare cases where it is really inconvenient to rewrite your code using 'if'. ## As last resort it's possible to implement the "?:" operator as function: ## def q(cond,on_true,on_false): ## from inspect import isfunction ## if cond: ## if not isfunction(on_true): return on_true ## else: return apply(on_true) ## else: ## if not isfunction(on_false): return on_false ## else: return apply(on_false) ## In most cases you'll pass b and c directly: q(a,b,c). If the wrong path shall not be interpreted encapsulate b and c within a lambda function: q(a,lambda: b, lambda: c). ## Edit this entry / Log info / Last changed on Sat Dec 1 14:35:14 2001 by Mathias Hasselmann ## -------------------------------------------------------------------------------- ## 4.17. My class defines __del__ but it is not called when I delete the object. ## There are several possible reasons for this. ## The del statement does not necessarily call __del__ -- it simply decrements the object's reference count, and if this reaches zero __del__ is called. ## If your data structures contain circular links (e.g. a tree where each child has a parent pointer and each parent has a list of children) the reference counts will never go back to zero. You'll have to define an explicit close() method which removes those pointers. Please don't ever call __del__ directly -- __del__ should call close() and close() should make sure that it can be called more than once for the same object. ## If the object has ever been a local variable (or argument, which is really the same thing) to a function that caught an expression in an except clause, chances are that a reference to the object still exists in that function's stack frame as contained in the stack trace. Normally, deleting (better: assigning None to) sys.exc_traceback will take care of this. If a stack was printed for an unhandled exception in an interactive interpreter, delete sys.last_traceback instead. ## There is code that deletes all objects when the interpreter exits, but it is not called if your Python has been configured to support threads (because other threads may still be active). You can define your own cleanup function using sys.exitfunc (see question 4.4). ## Finally, if your __del__ method raises an exception, a warning message is printed to sys.stderr. ## Starting with Python 2.0, a garbage collector periodically reclaims the space used by most cycles with no external references. (See the "gc" module documentation for details.) There are, however, pathological cases where it can be expected to fail. Moreover, the garbage collector runs some time after the last reference to your data structure vanishes, so your __del__ method may be called at an inconvenient and random time. This is inconvenient if you're trying to reproduce a problem. Worse, the order in which object's __del__ methods are executed is arbitrary. ## Another way to avoid cyclical references is to use the "weakref" module, which allows you to point to objects without incrementing their reference count. Tree data structures, for instance, should use weak references for their parent and sibling pointers (if they need them!). ## Question 6.14 is intended to explain the new garbage collection algorithm. ## Edit this entry / Log info / Last changed on Mon Jun 10 15:27:28 2002 by Matthias Urlichs ## -------------------------------------------------------------------------------- ## 4.18. How do I change the shell environment for programs called using os.popen() or os.system()? Changing os.environ doesn't work. ## You must be using either a version of python before 1.4, or on a (rare) system that doesn't have the putenv() library function. ## Before Python 1.4, modifying the environment passed to subshells was left out of the interpreter because there seemed to be no well-established portable way to do it (in particular, some systems, have putenv(), others have setenv(), and some have none at all). As of Python 1.4, almost all Unix systems do have putenv(), and so does the Win32 API, and thus the os module was modified so that changes to os.environ are trapped and the corresponding putenv() call is made. ## Edit this entry / Log info ## -------------------------------------------------------------------------------- ## 4.19. What is a class? ## A class is the particular object type created by executing a class statement. Class objects are used as templates, to create instance objects, which embody both the data structure (attributes) and program routines (methods) specific to a datatype. ## A class can be based on one or more other classes, called its base class(es). It then inherits the attributes and methods of its base classes. This allows an object model to be successively refined by inheritance. ## The term "classic class" is used to refer to the original class implementation in Python. One problem with classic classes is their inability to use the built-in data types (such as list and dictionary) as base classes. Starting with Python 2.2 an attempt is in progress to unify user-defined classes and built-in types. It is now possible to declare classes that inherit from built-in types. ## Edit this entry / Log info / Last changed on Mon May 27 01:31:21 2002 by Steve Holden ## -------------------------------------------------------------------------------- ## 4.20. What is a method? ## A method is a function that you normally call as x.name(arguments...) for some object x. The term is used for methods of classes and class instances as well as for methods of built-in objects. (The latter have a completely different implementation and only share the way their calls look in Python code.) Methods of classes (and class instances) are defined as functions inside the class definition. ## Edit this entry / Log info ## -------------------------------------------------------------------------------- ## 4.21. What is self? ## Self is merely a conventional name for the first argument of a method -- i.e. a function defined inside a class definition. A method defined as meth(self, a, b, c) should be called as x.meth(a, b, c) for some instance x of the class in which the definition occurs; the called method will think it is called as meth(x, a, b, c). ## Edit this entry / Log info ## -------------------------------------------------------------------------------- ## 4.22. What is an unbound method? ## An unbound method is a method defined in a class that is not yet bound to an instance. You get an unbound method if you ask for a class attribute that happens to be a function. You get a bound method if you ask for an instance attribute. A bound method knows which instance it belongs to and calling it supplies the instance automatically; an unbound method only knows which class it wants for its first argument (a derived class is also OK). Calling an unbound method doesn't "magically" derive the first argument from the context -- you have to provide it explicitly. ## Trivia note regarding bound methods: each reference to a bound method of a particular object creates a bound method object. If you have two such references (a = inst.meth; b = inst.meth), they will compare equal (a == b) but are not the same (a is not b). ## Edit this entry / Log info / Last changed on Wed May 6 18:07:25 1998 by Clarence Gardner ## -------------------------------------------------------------------------------- ## 4.23. How do I call a method defined in a base class from a derived class that overrides it? ## If your class definition starts with "class Derived(Base): ..." then you can call method meth defined in Base (or one of Base's base classes) as Base.meth(self, arguments...). Here, Base.meth is an unbound method (see previous question). ## Edit this entry / Log info ## -------------------------------------------------------------------------------- ## 4.24. How do I call a method from a base class without using the name of the base class? ## DON'T DO THIS. REALLY. I MEAN IT. It appears that you could call self.__class__.__bases__[0].meth(self, arguments...) but this fails when a doubly-derived method is derived from your class: for its instances, self.__class__.__bases__[0] is your class, not its base class -- so (assuming you are doing this from within Derived.meth) you would start a recursive call. ## Often when you want to do this you are forgetting that classes are first class in Python. You can "point to" the class you want to delegate an operation to either at the instance or at the subclass level. For example if you want to use a "glorp" operation of a superclass you can point to the right superclass to use. ## class subclass(superclass1, superclass2, superclass3): ## delegate_glorp = superclass2 ## ... ## def glorp(self, arg1, arg2): ## ... subclass specific stuff ... ## self.delegate_glorp.glorp(self, arg1, arg2) ## ... ## class subsubclass(subclass): ## delegate_glorp = superclass3 ## ... ## Note, however that setting delegate_glorp to subclass in subsubclass would cause an infinite recursion on subclass.delegate_glorp. Careful! Maybe you are getting too fancy for your own good. Consider simplifying the design (?). ## Edit this entry / Log info / Last changed on Mon Jul 28 13:58:22 1997 by aaron watters ## -------------------------------------------------------------------------------- ## 4.25. How can I organize my code to make it easier to change the base class? ## You could define an alias for the base class, assign the real base class to it before your class definition, and use the alias throughout your class. Then all you have to change is the value assigned to the alias. Incidentally, this trick is also handy if you want to decide dynamically (e.g. depending on availability of resources) which base class to use. Example: ## BaseAlias = ## class Derived(BaseAlias): ## def meth(self): ## BaseAlias.meth(self) ## ... ## Edit this entry / Log info / Last changed on Wed May 21 15:49:57 1997 by GvR ## -------------------------------------------------------------------------------- ## 4.26. How can I find the methods or attributes of an object? ## This depends on the object type. ## For an instance x of a user-defined class, instance attributes are found in the dictionary x.__dict__, and methods and attributes defined by its class are found in x.__class__.__bases__[i].__dict__ (for i in range(len(x.__class__.__bases__))). You'll have to walk the tree of base classes to find all class methods and attributes. ## Many, but not all built-in types define a list of their method names in x.__methods__, and if they have data attributes, their names may be found in x.__members__. However this is only a convention. ## For more information, read the source of the standard (but undocumented) module newdir. ## Edit this entry / Log info ## -------------------------------------------------------------------------------- ## 4.27. I can't seem to use os.read() on a pipe created with os.popen(). ## os.read() is a low-level function which takes a file descriptor (a small integer). os.popen() creates a high-level file object -- the same type used for sys.std{in,out,err} and returned by the builtin open() function. Thus, to read n bytes from a pipe p created with os.popen(), you need to use p.read(n). ## Edit this entry / Log info ## -------------------------------------------------------------------------------- ## 4.28. How can I create a stand-alone binary from a Python script? ## Even though there are Python compilers being developed, you probably don't need a real compiler, if all you want is a stand-alone program. There are three solutions to that. ## One is to use the freeze tool, which is included in the Python source tree as Tools/freeze. It converts Python byte code to C arrays. Using a C compiler, you can embed all your modules into a new program, which is then linked with the standard Python modules. ## It works by scanning your source recursively for import statements (in both forms) and looking for the modules in the standard Python path as well as in the source directory (for built-in modules). It then 1 the modules written in Python to C code (array initializers that can be turned into code objects using the marshal module) and creates a custom-made config file that only contains those built-in modules which are actually used in the program. It then compiles the generated C code and links it with the rest of the Python interpreter to form a self-contained binary which acts exactly like your script. ## (Hint: the freeze program only works if your script's filename ends in ".py".) ## There are several utilities which may be helpful. The first is Gordon McMillan's installer at ## http://www.mcmillan-inc.com/install1.html ## which works on Windows, Linux and at least some forms of Unix. ## Another is Thomas Heller's py2exe (Windows only) at ## http://starship.python.net/crew/theller/py2exe/ ## A third is Christian Tismer's SQFREEZE (http://starship.python.net/crew/pirx/) which appends the byte code to a specially-prepared Python interpreter, which will find the byte code in executable. ## A fourth is Fredrik Lundh's Squeeze (http://www.pythonware.com/products/python/squeeze/). ## Edit this entry / Log info / Last changed on Wed Jun 19 14:01:30 2002 by Gordon McMillan ## -------------------------------------------------------------------------------- ## 4.29. What WWW tools are there for Python? * ## See the chapters titled "Internet Protocols and Support" and "Internet Data Handling" in the Library Reference Manual. Python is full of good things which will help you build server-side and client-side web systems. ## A summary of available frameworks is maintained by Paul Boddie at ## http://thor.prohosting.com/~pboddie/Python/web_modules.html ## Cameron Laird maintains a useful set of pages about Python web technologies at ## http://starbase.neosoft.com/~claird/comp.lang.python/web_python.html/ ## There was a web browser written in Python, called Grail -- see http://sourceforge.net/project/grail/. This project has been terminated; http://cvs.sourceforge.net/cgi-bin/viewcvs.cgi/grail/grail/README gives more details. ## Edit this entry / Log info / Last changed on Mon Nov 11 22:48:25 2002 by GvR ## -------------------------------------------------------------------------------- ## 4.30. How do I run a subprocess with pipes connected to both input and output? ## Use the standard popen2 module. For example: ## import popen2 ## fromchild, tochild = popen2.popen2("command") ## tochild.write("input\n") ## tochild.flush() ## output = fromchild.readline() ## Warning: in general, it is unwise to do this, because you can easily cause a deadlock where your process is blocked waiting for output from the child, while the child is blocked waiting for input from you. This can be caused because the parent expects the child to output more text than it does, or it can be caused by data being stuck in stdio buffers due to lack of flushing. The Python parent can of course explicitly flush the data it sends to the child before it reads any output, but if the child is a naive C program it can easily have been written to never explicitly flush its output, even if it is interactive, since flushing is normally automatic. ## Note that a deadlock is also possible if you use popen3 to read stdout and stderr. If one of the two is too large for the internal buffer (increasing the buffersize does not help) and you read() the other one first, there is a deadlock, too. ## Note on a bug in popen2: unless your program calls wait() or waitpid(), finished child processes are never removed, and eventually calls to popen2 will fail because of a limit on the number of child processes. Calling os.waitpid with the os.WNOHANG option can prevent this; a good place to insert such a call would be before calling popen2 again. ## Another way to produce a deadlock: Call a wait() and there is still more output from the program than what fits into the internal buffers. ## In many cases, all you really need is to run some data through a command and get the result back. Unless the data is infinite in size, the easiest (and often the most efficient!) way to do this is to write it to a temporary file and run the command with that temporary file as input. The standard module tempfile exports a function mktemp() which generates unique temporary file names. ## import tempfile ## import os ## class Popen3: ## """ ## This is a deadlock-save version of popen, that returns ## an object with errorlevel, out (a string) and err (a string). ## (capturestderr may not work under windows.) ## Example: print Popen3('grep spam','\n\nhere spam\n\n').out ## """ ## def __init__(self,command,input=None,capturestderr=None): ## outfile=tempfile.mktemp() ## command="( %s ) > %s" % (command,outfile) ## if input: ## infile=tempfile.mktemp() ## open(infile,"w").write(input) ## command=command+" <"+infile ## if capturestderr: ## errfile=tempfile.mktemp() ## command=command+" 2>"+errfile ## self.errorlevel=os.system(command) >> 8 ## self.out=open(outfile,"r").read() ## os.remove(outfile) ## if input: ## os.remove(infile) ## if capturestderr: ## self.err=open(errfile,"r").read() ## os.remove(errfile) ## Note that many interactive programs (e.g. vi) don't work well with pipes substituted for standard input and output. You will have to use pseudo ttys ("ptys") instead of pipes. There is some undocumented code to use these in the library module pty.py -- I'm afraid you're on your own here. ## A different answer is a Python interface to Don Libes' "expect" library. A Python extension that interfaces to expect is called "expy" and available from http://expectpy.sourceforge.net/. ## A pure Python solution that works like expect is pexpect of Noah Spurrier. A beta version is available from http://pexpect.sourceforge.net/ ## Edit this entry / Log info / Last changed on Tue Sep 3 16:31:31 2002 by Tobias Polzin ## -------------------------------------------------------------------------------- ## 4.31. How do I call a function if I have the arguments in a tuple? ## Use the built-in function apply(). For instance, ## func(1, 2, 3) ## is equivalent to ## args = (1, 2, 3) ## apply(func, args) ## Note that func(args) is not the same -- it calls func() with exactly one argument, the tuple args, instead of three arguments, the integers 1, 2 and 3. ## In Python 2.0, you can also use extended call syntax: ## f(*args) is equivalent to apply(f, args) ## Edit this entry / Log info / Last changed on Tue Jan 2 03:42:50 2001 by Moshe Zadka ## -------------------------------------------------------------------------------- ## 4.32. How do I enable font-lock-mode for Python in Emacs? ## If you are using XEmacs 19.14 or later, any XEmacs 20, FSF Emacs 19.34 or any Emacs 20, font-lock should work automatically for you if you are using the latest python-mode.el. ## If you are using an older version of XEmacs or Emacs you will need to put this in your .emacs file: ## (defun my-python-mode-hook () ## (setq font-lock-keywords python-font-lock-keywords) ## (font-lock-mode 1)) ## (add-hook 'python-mode-hook 'my-python-mode-hook) ## Edit this entry / Log info / Last changed on Mon Apr 6 16:18:46 1998 by Barry Warsaw ## -------------------------------------------------------------------------------- ## 4.33. Is there a scanf() or sscanf() equivalent? ## Not as such. ## For simple input parsing, the easiest approach is usually to split the line into whitespace-delimited words using string.split(), and to convert decimal strings to numeric values using int(), long() or float(). (Python's int() is 32-bit and its long() is arbitrary precision.) string.split supports an optional "sep" parameter which is useful if the line uses something other than whitespace as a delimiter. ## For more complicated input parsing, regular expressions (see module re) are better suited and more powerful than C's sscanf(). ## There's a contributed module that emulates sscanf(), by Steve Clift; see contrib/Misc/sscanfmodule.c of the ftp site: ## http://www.python.org/ftp/python/contrib-09-Dec-1999/Misc/ ## Edit this entry / Log info / Last changed on Mon Jun 3 01:07:51 2002 by Neal Norwitz ## -------------------------------------------------------------------------------- ## 4.34. Can I have Tk events handled while waiting for I/O? ## Yes, and you don't even need threads! But you'll have to restructure your I/O code a bit. Tk has the equivalent of Xt's XtAddInput() call, which allows you to register a callback function which will be called from the Tk mainloop when I/O is possible on a file descriptor. Here's what you need: ## from Tkinter import tkinter ## tkinter.createfilehandler(file, mask, callback) ## The file may be a Python file or socket object (actually, anything with a fileno() method), or an integer file descriptor. The mask is one of the constants tkinter.READABLE or tkinter.WRITABLE. The callback is called as follows: ## callback(file, mask) ## You must unregister the callback when you're done, using ## tkinter.deletefilehandler(file) ## Note: since you don't know *how many bytes* are available for reading, you can't use the Python file object's read or readline methods, since these will insist on reading a predefined number of bytes. For sockets, the recv() or recvfrom() methods will work fine; for other files, use os.read(file.fileno(), maxbytecount). ## Edit this entry / Log info ## -------------------------------------------------------------------------------- ## 4.35. How do I write a function with output parameters (call by reference)? ## [Mark Lutz] The thing to remember is that arguments are passed by assignment in Python. Since assignment just creates references to objects, there's no alias between an argument name in the caller and callee, and so no call-by-reference per se. But you can simulate it in a number of ways: ## 1) By using global variables; but you probably shouldn't :-) ## 2) By passing a mutable (changeable in-place) object: ## def func1(a): ## a[0] = 'new-value' # 'a' references a mutable list ## a[1] = a[1] + 1 # changes a shared object ## args = ['old-value', 99] ## func1(args) ## print args[0], args[1] # output: new-value 100 ## 3) By returning a tuple, holding the final values of arguments: ## def func2(a, b): ## a = 'new-value' # a and b are local names ## b = b + 1 # assigned to new objects ## return a, b # return new values ## x, y = 'old-value', 99 ## x, y = func2(x, y) ## print x, y # output: new-value 100 ## 4) And other ideas that fall-out from Python's object model. For instance, it might be clearer to pass in a mutable dictionary: ## def func3(args): ## args['a'] = 'new-value' # args is a mutable dictionary ## args['b'] = args['b'] + 1 # change it in-place ## args = {'a':' old-value', 'b': 99} ## func3(args) ## print args['a'], args['b'] ## 5) Or bundle-up values in a class instance: ## class callByRef: ## def __init__(self, **args): ## for (key, value) in args.items(): ## setattr(self, key, value) ## def func4(args): ## args.a = 'new-value' # args is a mutable callByRef ## args.b = args.b + 1 # change object in-place ## args = callByRef(a='old-value', b=99) ## func4(args) ## print args.a, args.b ## But there's probably no good reason to get this complicated :-). ## [Python's author favors solution 3 in most cases.] ## Edit this entry / Log info / Last changed on Sun Jun 8 23:49:46 1997 by David Ascher ## -------------------------------------------------------------------------------- ## 4.36. Please explain the rules for local and global variables in Python. ## [Ken Manheimer] In Python, procedure variables are implicitly global, unless they are assigned anywhere within the block. In that case they are implicitly local, and you need to explicitly declare them as 'global'. ## Though a bit surprising at first, a moment's consideration explains this. On one hand, requirement of 'global' for assigned vars provides a bar against unintended side-effects. On the other hand, if global were required for all global references, you'd be using global all the time. Eg, you'd have to declare as global every reference to a builtin function, or to a component of an imported module. This clutter would defeat the usefulness of the 'global' declaration for identifying side-effects. ## Edit this entry / Log info / Last changed on Fri Aug 28 09:53:27 1998 by GvR ## -------------------------------------------------------------------------------- ## 4.37. How can I have modules that mutually import each other? ## Suppose you have the following modules: ## foo.py: ## from bar import bar_var ## foo_var=1 ## bar.py: ## from foo import foo_var ## bar_var=2 ## The problem is that the above is processed by the interpreter thus: ## main imports foo ## Empty globals for foo are created ## foo is compiled and starts executing ## foo imports bar ## Empty globals for bar are created ## bar is compiled and starts executing ## bar imports foo (which is a no-op since there already is a module named foo) ## bar.foo_var = foo.foo_var ## ... ## The last step fails, because Python isn't done with interpreting foo yet and the global symbol dict for foo is still empty. ## The same thing happens when you use "import foo", and then try to access "foo.one" in global code. ## There are (at least) three possible workarounds for this problem. ## Guido van Rossum recommends to avoid all uses of "from import ..." (so everything from an imported module is referenced as .) and to place all code inside functions. Initializations of global variables and class variables should use constants or built-in functions only. ## Jim Roskind suggests the following order in each module: ## exports (globals, functions, and classes that don't need imported base classes) ## import statements ## active code (including globals that are initialized from imported values). ## Python's author doesn't like this approach much because the imports appear in a strange place, but has to admit that it works. ## Matthias Urlichs recommends to restructure your code so that the recursive import is not necessary in the first place. ## These solutions are not mutually exclusive. ## Edit this entry / Log info / Last changed on Mon Jun 3 06:52:51 2002 by Matthias Urlichs ## -------------------------------------------------------------------------------- ## 4.38. How do I copy an object in Python? ## Try copy.copy() or copy.deepcopy() for the general case. Not all objects can be copied, but most can. ## Dictionaries have a copy method. Sequences can be copied by slicing: ## new_l = l[:] ## Edit this entry / Log info / Last changed on Thu Mar 21 05:40:26 2002 by Erno Kuusela ## -------------------------------------------------------------------------------- ## 4.39. How to implement persistent objects in Python? (Persistent == automatically saved to and restored from disk.) ## The library module "pickle" now solves this in a very general way (though you still can't store things like open files, sockets or windows), and the library module "shelve" uses pickle and (g)dbm to create persistent mappings containing arbitrary Python objects. For possibly better performance also look for the latest version of the relatively recent cPickle module. ## A more awkward way of doing things is to use pickle's little sister, marshal. The marshal module provides very fast ways to store noncircular basic Python types to files and strings, and back again. Although marshal does not do fancy things like store instances or handle shared references properly, it does run extremely fast. For example loading a half megabyte of data may take less than a third of a second (on some machines). This often beats doing something more complex and general such as using gdbm with pickle/shelve. ## Edit this entry / Log info / Last changed on Sun Jun 8 22:59:00 1997 by David Ascher ## -------------------------------------------------------------------------------- ## 4.40. I try to use __spam and I get an error about _SomeClassName__spam. ## Variables with double leading underscore are "mangled" to provide a simple but effective way to define class private variables. See the chapter "New in Release 1.4" in the Python Tutorial. ## Edit this entry / Log info ## -------------------------------------------------------------------------------- ## 4.41. How do I delete a file? And other file questions. ## Use os.remove(filename) or os.unlink(filename); for documentation, see the posix section of the library manual. They are the same, unlink() is simply the Unix name for this function. In earlier versions of Python, only os.unlink() was available. ## To remove a directory, use os.rmdir(); use os.mkdir() to create one. ## To rename a file, use os.rename(). ## To truncate a file, open it using f = open(filename, "r+"), and use f.truncate(offset); offset defaults to the current seek position. (The "r+" mode opens the file for reading and writing.) There's also os.ftruncate(fd, offset) for files opened with os.open() -- for advanced Unix hacks only. ## The shutil module also contains a number of functions to work on files including copyfile, copytree, and rmtree amongst others. ## Edit this entry / Log info / Last changed on Thu Dec 28 12:30:01 2000 by Bjorn Pettersen ## -------------------------------------------------------------------------------- ## 4.42. How to modify urllib or httplib to support HTTP/1.1? ## Recent versions of Python (2.0 and onwards) support HTTP/1.1 natively. ## Edit this entry / Log info / Last changed on Tue Jan 2 02:56:56 2001 by Moshe Zadka ## -------------------------------------------------------------------------------- ## 4.43. Unexplicable syntax errors in compile() or exec. ## When a statement suite (as opposed to an expression) is compiled by compile(), exec or execfile(), it must end in a newline. In some cases, when the source ends in an indented block it appears that at least two newlines are required. ## Edit this entry / Log info ## -------------------------------------------------------------------------------- ## 4.44. How do I convert a string to a number? ## For integers, use the built-in int() function, e.g. int('144') == 144. Similarly, long() converts from string to long integer, e.g. long('144') == 144L; and float() to floating-point, e.g. float('144') == 144.0. ## Note that these are restricted to decimal interpretation, so that int('0144') == 144 and int('0x144') raises ValueError. For Python 2.0 int takes the base to convert from as a second optional argument, so int('0x144', 16) == 324. ## For greater flexibility, or before Python 1.5, import the module string and use the string.atoi() function for integers, string.atol() for long integers, or string.atof() for floating-point. E.g., string.atoi('100', 16) == string.atoi('0x100', 0) == 256. See the library reference manual section for the string module for more details. ## While you could use the built-in function eval() instead of any of those, this is not recommended, because someone could pass you a Python expression that might have unwanted side effects (like reformatting your disk). It also has the effect of interpreting numbers as Python expressions, so that e.g. eval('09') gives a syntax error since Python regards numbers starting with '0' as octal (base 8). ## Edit this entry / Log info / Last changed on Thu Dec 28 12:37:34 2000 by Bjorn Pettersen ## -------------------------------------------------------------------------------- ## 4.45. How do I convert a number to a string? ## To convert, e.g., the number 144 to the string '144', use the built-in function repr() or the backquote notation (these are equivalent). If you want a hexadecimal or octal representation, use the built-in functions hex() or oct(), respectively. For fancy formatting, use the % operator on strings, just like C printf formats, e.g. "%04d" % 144 yields '0144' and "%.3f" % (1/3.0) yields '0.333'. See the library reference manual for details. ## Edit this entry / Log info ## -------------------------------------------------------------------------------- ## 4.46. How do I copy a file? ## There's the shutil module which contains a copyfile() function that implements a copy loop; it isn't good enough for the Macintosh, though: it doesn't copy the resource fork and Finder info. ## Edit this entry / Log info / Last changed on Tue Jan 2 02:59:40 2001 by Moshe Zadka ## -------------------------------------------------------------------------------- ## 4.47. How do I check if an object is an instance of a given class or of a subclass of it? ## If you are developing the classes from scratch it might be better to program in a more proper object-oriented style -- instead of doing a different thing based on class membership, why not use a method and define the method differently in different classes? ## However, there are some legitimate situations where you need to test for class membership. ## In Python 1.5, you can use the built-in function isinstance(obj, cls). ## The following approaches can be used with earlier Python versions: ## An unobvious method is to raise the object as an exception and to try to catch the exception with the class you're testing for: ## def is_instance_of(the_instance, the_class): ## try: ## raise the_instance ## except the_class: ## return 1 ## except: ## return 0 ## This technique can be used to distinguish "subclassness" from a collection of classes as well ## try: ## raise the_instance ## except Audible: ## the_instance.play(largo) ## except Visual: ## the_instance.display(gaudy) ## except Olfactory: ## sniff(the_instance) ## except: ## raise ValueError, "dunno what to do with this!" ## This uses the fact that exception catching tests for class or subclass membership. ## A different approach is to test for the presence of a class attribute that is presumably unique for the given class. For instance: ## class MyClass: ## ThisIsMyClass = 1 ## ... ## def is_a_MyClass(the_instance): ## return hasattr(the_instance, 'ThisIsMyClass') ## This version is easier to inline, and probably faster (inlined it is definitely faster). The disadvantage is that someone else could cheat: ## class IntruderClass: ## ThisIsMyClass = 1 # Masquerade as MyClass ## ... ## but this may be seen as a feature (anyway, there are plenty of other ways to cheat in Python). Another disadvantage is that the class must be prepared for the membership test. If you do not "control the source code" for the class it may not be advisable to modify the class to support testability. ## Edit this entry / Log info / Last changed on Fri Jan 2 15:16:04 1998 by GvR ## -------------------------------------------------------------------------------- ## 4.48. What is delegation? ## Delegation refers to an object oriented technique Python programmers may implement with particular ease. Consider the following: ## from string import upper ## class UpperOut: ## def __init__(self, outfile): ## self.__outfile = outfile ## def write(self, str): ## self.__outfile.write( upper(str) ) ## def __getattr__(self, name): ## return getattr(self.__outfile, name) ## Here the UpperOut class redefines the write method to convert the argument string to upper case before calling the underlying self.__outfile.write method, but all other methods are delegated to the underlying self.__outfile object. The delegation is accomplished via the "magic" __getattr__ method. Please see the language reference for more information on the use of this method. ## Note that for more general cases delegation can get trickier. Particularly when attributes must be set as well as gotten the class must define a __settattr__ method too, and it must do so carefully. ## The basic implementation of __setattr__ is roughly equivalent to the following: ## class X: ## ... ## def __setattr__(self, name, value): ## self.__dict__[name] = value ## ... ## Most __setattr__ implementations must modify self.__dict__ to store local state for self without causing an infinite recursion. ## Edit this entry / Log info / Last changed on Wed Aug 13 07:11:24 1997 by aaron watters ## -------------------------------------------------------------------------------- ## 4.49. How do I test a Python program or component. ## We presume for the purposes of this question you are interested in standalone testing, rather than testing your components inside a testing framework. The best-known testing framework for Python is the PyUnit module, maintained at ## http://pyunit.sourceforge.net/ ## For standalone testing, it helps to write the program so that it may be easily tested by using good modular design. In particular your program should have almost all functionality encapsulated in either functions or class methods -- and this sometimes has the surprising and delightful effect of making the program run faster (because local variable accesses are faster than global accesses). Furthermore the program should avoid depending on mutating global variables, since this makes testing much more difficult to do. ## The "global main logic" of your program may be as simple as ## if __name__=="__main__": ## main_logic() ## at the bottom of the main module of your program. ## Once your program is organized as a tractable collection of functions and class behaviours you should write test functions that exercise the behaviours. A test suite can be associated with each module which automates a sequence of tests. This sounds like a lot of work, but since Python is so terse and flexible it's surprisingly easy. You can make coding much more pleasant and fun by writing your test functions in parallel with the "production code", since this makes it easy to find bugs and even design flaws earlier. ## "Support modules" that are not intended to be the main module of a program may include a "test script interpretation" which invokes a self test of the module. ## if __name__ == "__main__": ## self_test() ## Even programs that interact with complex external interfaces may be tested when the external interfaces are unavailable by using "fake" interfaces implemented in Python. For an example of a "fake" interface, the following class defines (part of) a "fake" file interface: ## import string ## testdata = "just a random sequence of characters" ## class FakeInputFile: ## data = testdata ## position = 0 ## closed = 0 ## def read(self, n=None): ## self.testclosed() ## p = self.position ## if n is None: ## result= self.data[p:] ## else: ## result= self.data[p: p+n] ## self.position = p + len(result) ## return result ## def seek(self, n, m=0): ## self.testclosed() ## last = len(self.data) ## p = self.position ## if m==0: ## final=n ## elif m==1: ## final=n+p ## elif m==2: ## final=len(self.data)+n ## else: ## raise ValueError, "bad m" ## if final<0: ## raise IOError, "negative seek" ## self.position = final ## def isatty(self): ## return 0 ## def tell(self): ## return self.position ## def close(self): ## self.closed = 1 ## def testclosed(self): ## if self.closed: ## raise IOError, "file closed" ## Try f=FakeInputFile() and test out its operations. ## Edit this entry / Log info / Last changed on Mon Jun 3 01:12:10 2002 by Neal Norwitz ## -------------------------------------------------------------------------------- ## 4.50. My multidimensional list (array) is broken! What gives? ## You probably tried to make a multidimensional array like this. ## A = [[None] * 2] * 3 ## This makes a list containing 3 references to the same list of length two. Changes to one row will show in all rows, which is probably not what you want. The following works much better: ## A = [None]*3 ## for i in range(3): ## A[i] = [None] * 2 ## This generates a list containing 3 different lists of length two. ## If you feel weird, you can also do it in the following way: ## w, h = 2, 3 ## A = map(lambda i,w=w: [None] * w, range(h)) ## For Python 2.0 the above can be spelled using a list comprehension: ## w,h = 2,3 ## A = [ [None]*w for i in range(h) ] ## Edit this entry / Log info / Last changed on Thu Dec 28 12:18:35 2000 by Bjorn Pettersen ## -------------------------------------------------------------------------------- ## 4.51. I want to do a complicated sort: can you do a Schwartzian Transform in Python? ## Yes, and in Python you only have to write it once: ## def st(List, Metric): ## def pairing(element, M = Metric): ## return (M(element), element) ## paired = map(pairing, List) ## paired.sort() ## return map(stripit, paired) ## def stripit(pair): ## return pair[1] ## This technique, attributed to Randal Schwartz, sorts the elements of a list by a metric which maps each element to its "sort value". For example, if L is a list of string then ## import string ## Usorted = st(L, string.upper) ## def intfield(s): ## return string.atoi( string.strip(s[10:15] ) ) ## Isorted = st(L, intfield) ## Usorted gives the elements of L sorted as if they were upper case, and Isorted gives the elements of L sorted by the integer values that appear in the string slices starting at position 10 and ending at position 15. In Python 2.0 this can be done more naturally with list comprehensions: ## tmp1 = [ (x.upper(), x) for x in L ] # Schwartzian transform ## tmp1.sort() ## Usorted = [ x[1] for x in tmp1 ] ## tmp2 = [ (int(s[10:15]), s) for s in L ] # Schwartzian transform ## tmp2.sort() ## Isorted = [ x[1] for x in tmp2 ] ## Note that Isorted may also be computed by ## def Icmp(s1, s2): ## return cmp( intfield(s1), intfield(s2) ) ## Isorted = L[:] ## Isorted.sort(Icmp) ## but since this method computes intfield many times for each element of L, it is slower than the Schwartzian Transform. ## Edit this entry / Log info / Last changed on Sat Jun 1 19:18:46 2002 by Neal Norwitz ## -------------------------------------------------------------------------------- ## 4.52. How to convert between tuples and lists? ## The function tuple(seq) converts any sequence into a tuple with the same items in the same order. For example, tuple([1, 2, 3]) yields (1, 2, 3) and tuple('abc') yields ('a', 'b', 'c'). If the argument is a tuple, it does not make a copy but returns the same object, so it is cheap to call tuple() when you aren't sure that an object is already a tuple. ## The function list(seq) converts any sequence into a list with the same items in the same order. For example, list((1, 2, 3)) yields [1, 2, 3] and list('abc') yields ['a', 'b', 'c']. If the argument is a list, it makes a copy just like seq[:] would. ## Edit this entry / Log info / Last changed on Sun Jun 14 14:18:53 1998 by Tim Peters ## -------------------------------------------------------------------------------- ## 4.53. Files retrieved with urllib contain leading garbage that looks like email headers. ## Extremely old versions of Python supplied libraries which did not support HTTP/1.1; the vanilla httplib in Python 1.4 only recognized HTTP/1.0. In Python 2.0 full HTTP/1.1 support is included. ## Edit this entry / Log info / Last changed on Mon Jan 8 17:26:18 2001 by Steve Holden ## -------------------------------------------------------------------------------- ## 4.54. How do I get a list of all instances of a given class? ## Python does not keep track of all instances of a class (or of a built-in type). ## You can program the class's constructor to keep track of all instances, but unless you're very clever, this has the disadvantage that the instances never get deleted,because your list of all instances keeps a reference to them. ## (The trick is to regularly inspect the reference counts of the instances you've retained, and if the reference count is below a certain level, remove it from the list. Determining that level is tricky -- it's definitely larger than 1.) ## Edit this entry / Log info / Last changed on Tue May 27 23:52:16 1997 by GvR ## -------------------------------------------------------------------------------- ## 4.55. A regular expression fails with regex.error: match failure. ## This is usually caused by too much backtracking; the regular expression engine has a fixed size stack which holds at most 4000 backtrack points. Every character matched by e.g. ".*" accounts for a backtrack point, so even a simple search like ## regex.match('.*x',"x"*5000) ## will fail. ## This is fixed in the re module introduced with Python 1.5; consult the Library Reference section on re for more information. ## Edit this entry / Log info / Last changed on Thu Jul 30 12:35:49 1998 by A.M. Kuchling ## -------------------------------------------------------------------------------- ## 4.56. I can't get signal handlers to work. ## The most common problem is that the signal handler is declared with the wrong argument list. It is called as ## handler(signum, frame) ## so it should be declared with two arguments: ## def handler(signum, frame): ## ... ## Edit this entry / Log info / Last changed on Wed May 28 09:29:08 1997 by GvR ## -------------------------------------------------------------------------------- ## 4.57. I can't use a global variable in a function? Help! ## Did you do something like this? ## x = 1 # make a global ## def f(): ## print x # try to print the global ## ... ## for j in range(100): ## if q>3: ## x=4 ## Any variable assigned in a function is local to that function. unless it is specifically declared global. Since a value is bound to x as the last statement of the function body, the compiler assumes that x is local. Consequently the "print x" attempts to print an uninitialized local variable and will trigger a NameError. ## In such cases the solution is to insert an explicit global declaration at the start of the function, making it ## def f(): ## global x ## print x # try to print the global ## ... ## for j in range(100): ## if q>3: ## x=4 ## In this case, all references to x are interpreted as references to the x from the module namespace. ## Edit this entry / Log info / Last changed on Mon Feb 12 15:52:12 2001 by Steve Holden ## -------------------------------------------------------------------------------- ## 4.58. What's a negative index? Why doesn't list.insert() use them? ## Python sequences are indexed with positive numbers and negative numbers. For positive numbers 0 is the first index 1 is the second index and so forth. For negative indices -1 is the last index and -2 is the pentultimate (next to last) index and so forth. Think of seq[-n] as the same as seq[len(seq)-n]. ## Using negative indices can be very convenient. For example if the string Line ends in a newline then Line[:-1] is all of Line except the newline. ## Sadly the list builtin method L.insert does not observe negative indices. This feature could be considered a mistake but since existing programs depend on this feature it may stay around forever. L.insert for negative indices inserts at the start of the list. To get "proper" negative index behaviour use L[n:n] = [x] in place of the insert method. ## Edit this entry / Log info / Last changed on Wed Aug 13 07:03:18 1997 by aaron watters ## -------------------------------------------------------------------------------- ## 4.59. How can I sort one list by values from another list? ## You can sort lists of tuples. ## >>> list1 = ["what", "I'm", "sorting", "by"] ## >>> list2 = ["something", "else", "to", "sort"] ## >>> pairs = map(None, list1, list2) ## >>> pairs ## [('what', 'something'), ("I'm", 'else'), ('sorting', 'to'), ('by', 'sort')] ## >>> pairs.sort() ## >>> pairs ## [("I'm", 'else'), ('by', 'sort'), ('sorting', 'to'), ('what', 'something')] ## >>> result = pairs[:] ## >>> for i in xrange(len(result)): result[i] = result[i][1] ## ... ## >>> result ## ['else', 'sort', 'to', 'something'] ## And if you didn't understand the question, please see the example above ;c). Note that "I'm" sorts before "by" because uppercase "I" comes before lowercase "b" in the ascii order. Also see 4.51. ## In Python 2.0 this can be done like: ## >>> list1 = ["what", "I'm", "sorting", "by"] ## >>> list2 = ["something", "else", "to", "sort"] ## >>> pairs = zip(list1, list2) ## >>> pairs ## [('what', 'something'), ("I'm", 'else'), ('sorting', 'to'), ('by', 'sort')] ## >>> pairs.sort() ## >>> result = [ x[1] for x in pairs ] ## >>> result ## ['else', 'sort', 'to', 'something'] ## [Followup] ## Someone asked, why not this for the last steps: ## result = [] ## for p in pairs: result.append(p[1]) ## This is much more legible. However, a quick test shows that it is almost twice as slow for long lists. Why? First of all, the append() operation has to reallocate memory, and while it uses some tricks to avoid doing that each time, it still has to do it occasionally, and apparently that costs quite a bit. Second, the expression "result.append" requires an extra attribute lookup. The attribute lookup could be done away with by rewriting as follows: ## result = [] ## append = result.append ## for p in pairs: append(p[1]) ## which gains back some speed, but is still considerably slower than the original solution, and hardly less convoluted. ## Edit this entry / Log info / Last changed on Thu Dec 28 12:56:35 2000 by Bjorn Pettersen ## -------------------------------------------------------------------------------- ## 4.60. Why doesn't dir() work on builtin types like files and lists? ## It does starting with Python 1.5. ## Using 1.4, you can find out which methods a given object supports by looking at its __methods__ attribute: ## >>> List = [] ## >>> List.__methods__ ## ['append', 'count', 'index', 'insert', 'remove', 'reverse', 'sort'] ## Edit this entry / Log info / Last changed on Thu Sep 16 14:56:42 1999 by Skip Montanaro ## -------------------------------------------------------------------------------- ## 4.61. How can I mimic CGI form submission (METHOD=POST)? ## I would like to retrieve web pages that are the result of POSTing a form. Is there existing code that would let me do this easily? ## Yes. Here's a simple example that uses httplib. ## #!/usr/local/bin/python ## import httplib, sys, time ## ### build the query string ## qs = "First=Josephine&MI=Q&Last=Public" ## ### connect and send the server a path ## httpobj = httplib.HTTP('www.some-server.out-there', 80) ## httpobj.putrequest('POST', '/cgi-bin/some-cgi-script') ## ### now generate the rest of the HTTP headers... ## httpobj.putheader('Accept', '*/*') ## httpobj.putheader('Connection', 'Keep-Alive') ## httpobj.putheader('Content-type', 'application/x-www-form-urlencoded') ## httpobj.putheader('Content-length', '%d' % len(qs)) ## httpobj.endheaders() ## httpobj.send(qs) ## ### find out what the server said in response... ## reply, msg, hdrs = httpobj.getreply() ## if reply != 200: ## sys.stdout.write(httpobj.getfile().read()) ## Note that in general for "url encoded posts" (the default) query strings must be "quoted" to, for example, change equals signs and spaces to an encoded form when they occur in name or value. Use urllib.quote to perform this quoting. For example to send name="Guy Steele, Jr.": ## >>> from urllib import quote ## >>> x = quote("Guy Steele, Jr.") ## >>> x ## 'Guy%20Steele,%20Jr.' ## >>> query_string = "name="+x ## >>> query_string ## 'name=Guy%20Steele,%20Jr.' ## Edit this entry / Log info / Last changed on Mon Jun 21 03:47:07 1999 by TAB ## -------------------------------------------------------------------------------- ## 4.62. If my program crashes with a bsddb (or anydbm) database open, it gets corrupted. How come? ## Databases opened for write access with the bsddb module (and often by the anydbm module, since it will preferentially use bsddb) must explicitly be closed using the close method of the database. The underlying libdb package caches database contents which need to be converted to on-disk form and written, unlike regular open files which already have the on-disk bits in the kernel's write buffer, where they can just be dumped by the kernel with the program exits. ## If you have initialized a new bsddb database but not written anything to it before the program crashes, you will often wind up with a zero-length file and encounter an exception the next time the file is opened. ## Edit this entry / Log info / Last changed on Mon Jun 3 01:15:01 2002 by Neal Norwitz ## -------------------------------------------------------------------------------- ## 4.63. How do I make a Python script executable on Unix? ## You need to do two things: the script file's mode must be executable (include the 'x' bit), and the first line must begin with #! followed by the pathname for the Python interpreter. ## The first is done by executing 'chmod +x scriptfile' or perhaps 'chmod 755 scriptfile'. ## The second can be done in a number of way. The most straightforward way is to write ## #!/usr/local/bin/python ## as the very first line of your file - or whatever the pathname is where the python interpreter is installed on your platform. ## If you would like the script to be independent of where the python interpreter lives, you can use the "env" program. On almost all platforms, the following will work, assuming the python interpreter is in a directory on the user's $PATH: ## #! /usr/bin/env python ## Note -- *don't* do this for CGI scripts. The $PATH variable for CGI scripts is often very minimal, so you need to use the actual absolute pathname of the interpreter. ## Occasionally, a user's environment is so full that the /usr/bin/env program fails; or there's no env program at all. In that case, you can try the following hack (due to Alex Rezinsky): ## #! /bin/sh ## """:" ## exec python $0 ${1+"$@"} ## """ ## The disadvantage is that this defines the script's __doc__ string. However, you can fix that by adding ## __doc__ = """...Whatever...""" ## Edit this entry / Log info / Last changed on Mon Jan 15 09:19:16 2001 by Neal Norwitz ## -------------------------------------------------------------------------------- ## 4.64. How do you remove duplicates from a list? ## See the Python Cookbook for a long discussion of many cool ways: ## http://aspn.activestate.com/ASPN/Cookbook/Python/Recipe/52560 ## Generally, if you don't mind reordering the List ## if List: ## List.sort() ## last = List[-1] ## for i in range(len(List)-2, -1, -1): ## if last==List[i]: del List[i] ## else: last=List[i] ## If all elements of the list may be used as dictionary keys (ie, they are all hashable) this is often faster ## d = {} ## for x in List: d[x]=x ## List = d.values() ## Also, for extremely large lists you might consider more optimal alternatives to the first one. The second one is pretty good whenever it can be used. ## Edit this entry / Log info / Last changed on Fri May 24 21:56:33 2002 by Tim Peters ## -------------------------------------------------------------------------------- ## 4.65. Are there any known year 2000 problems in Python? ## I am not aware of year 2000 deficiencies in Python 1.5. Python does very few date calculations and for what it does, it relies on the C library functions. Python generally represent times either as seconds since 1970 or as a tuple (year, month, day, ...) where the year is expressed with four digits, which makes Y2K bugs unlikely. So as long as your C library is okay, Python should be okay. Of course, I cannot vouch for your Python code! ## Given the nature of freely available software, I have to add that this statement is not legally binding. The Python copyright notice contains the following disclaimer: ## STICHTING MATHEMATISCH CENTRUM AND CNRI DISCLAIM ALL WARRANTIES WITH ## REGARD TO THIS SOFTWARE, INCLUDING ALL IMPLIED WARRANTIES OF ## MERCHANTABILITY AND FITNESS, IN NO EVENT SHALL STICHTING MATHEMATISCH ## CENTRUM OR CNRI BE LIABLE FOR ANY SPECIAL, INDIRECT OR CONSEQUENTIAL ## DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR ## PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR OTHER ## TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR ## PERFORMANCE OF THIS SOFTWARE. ## The good news is that if you encounter a problem, you have full source available to track it down and fix it! ## Edit this entry / Log info / Last changed on Fri Apr 10 14:59:31 1998 by GvR ## -------------------------------------------------------------------------------- ## 4.66. I want a version of map that applies a method to a sequence of objects! Help! ## Get fancy! ## def method_map(objects, method, arguments): ## """method_map([a,b], "flog", (1,2)) gives [a.flog(1,2), b.flog(1,2)]""" ## nobjects = len(objects) ## methods = map(getattr, objects, [method]*nobjects) ## return map(apply, methods, [arguments]*nobjects) ## It's generally a good idea to get to know the mysteries of map and apply and getattr and the other dynamic features of Python. ## Edit this entry / Log info / Last changed on Mon Jan 5 14:21:14 1998 by Aaron Watters ## -------------------------------------------------------------------------------- ## 4.67. How do I generate random numbers in Python? ## The standard library module "random" implements a random number generator. Usage is simple: ## import random ## random.random() ## This returns a random floating point number in the range [0, 1). ## There are also many other specialized generators in this module, such as ## randrange(a, b) chooses an integer in the range [a, b) ## uniform(a, b) chooses a floating point number in the range [a, b) ## normalvariate(mean, sdev) sample from normal (Gaussian) distribution ## Some higher-level functions operate on sequences directly, such as ## choice(S) chooses random element from a given sequence ## shuffle(L) shuffles a list in-place, i.e. permutes it randomly ## There's also a class, Random, which you can instantiate to create independent multiple random number generators. ## All this is documented in the library reference manual. Note that the module "whrandom" is obsolete. ## Edit this entry / Log info / Last changed on Mon Jun 3 01:16:51 2002 by Neal Norwitz ## -------------------------------------------------------------------------------- ## 4.68. How do I access the serial (RS232) port? ## There's a Windows serial communication module (for communication over RS 232 serial ports) at ## ftp://ftp.python.org/pub/python/contrib/sio-151.zip ## http://www.python.org/ftp/python/contrib/sio-151.zip ## For DOS, try Hans Nowak's Python-DX, which supports this, at: ## http://www.cuci.nl/~hnowak/ ## For Unix, see a usenet post by Mitch Chapman: ## http://groups.google.com/groups?selm=34A04430.CF9@ohioee.com ## For Win32, POSIX(Linux, BSD, *), Jython, Chris': ## http://pyserial.sourceforge.net ## Edit this entry / Log info / Last changed on Tue Jul 2 21:11:07 2002 by Chris Liechti ## -------------------------------------------------------------------------------- ## 4.69. Images on Tk-Buttons don't work in Py15? ## They do work, but you must keep your own reference to the image object now. More verbosely, you must make sure that, say, a global variable or a class attribute refers to the object. ## Quoting Fredrik Lundh from the mailinglist: ## Well, the Tk button widget keeps a reference to the internal ## photoimage object, but Tkinter does not. So when the last ## Python reference goes away, Tkinter tells Tk to release the ## photoimage. But since the image is in use by a widget, Tk ## doesn't destroy it. Not completely. It just blanks the image, ## making it completely transparent... ## And yes, there was a bug in the keyword argument handling ## in 1.4 that kept an extra reference around in some cases. And ## when Guido fixed that bug in 1.5, he broke quite a few Tkinter ## programs... ## Edit this entry / Log info / Last changed on Tue Feb 3 11:31:03 1998 by Case Roole ## -------------------------------------------------------------------------------- ## 4.70. Where is the math.py (socket.py, regex.py, etc.) source file? ## If you can't find a source file for a module it may be a builtin or dynamically loaded module implemented in C, C++ or other compiled language. In this case you may not have the source file or it may be something like mathmodule.c, somewhere in a C source directory (not on the Python Path). ## Fredrik Lundh (fredrik@pythonware.com) explains (on the python-list): ## There are (at least) three kinds of modules in Python: 1) modules written in Python (.py); 2) modules written in C and dynamically loaded (.dll, .pyd, .so, .sl, etc); 3) modules written in C and linked with the interpreter; to get a list of these, type: ## import sys ## print sys.builtin_module_names ## Edit this entry / Log info / Last changed on Tue Feb 3 13:55:33 1998 by Aaron Watters ## -------------------------------------------------------------------------------- ## 4.71. How do I send mail from a Python script? ## The standard library module smtplib does this. Here's a very simple interactive mail sender that uses it. This method will work on any host that supports an SMTP listener. ## import sys, smtplib ## fromaddr = raw_input("From: ") ## toaddrs = raw_input("To: ").split(',') ## print "Enter message, end with ^D:" ## msg = '' ## while 1: ## line = sys.stdin.readline() ## if not line: ## break ## msg = msg + line ## # The actual mail send ## server = smtplib.SMTP('localhost') ## server.sendmail(fromaddr, toaddrs, msg) ## server.quit() ## If the local host doesn't have an SMTP listener, you need to find one. The simple method is to ask the user. Alternately, you can use the DNS system to find the mail gateway(s) responsible for the source address. ## A Unix-only alternative uses sendmail. The location of the sendmail program varies between systems; sometimes it is /usr/lib/sendmail, sometime /usr/sbin/sendmail. The sendmail manual page will help you out. Here's some sample code: ## SENDMAIL = "/usr/sbin/sendmail" # sendmail location ## import os ## p = os.popen("%s -t -i" % SENDMAIL, "w") ## p.write("To: cary@ratatosk.org\n") ## p.write("Subject: test\n") ## p.write("\n") # blank line separating headers from body ## p.write("Some text\n") ## p.write("some more text\n") ## sts = p.close() ## if sts != 0: ## print "Sendmail exit status", sts ## Edit this entry / Log info / Last changed on Mon Jun 3 07:05:12 2002 by Matthias Urlichs ## -------------------------------------------------------------------------------- ## 4.72. How do I avoid blocking in connect() of a socket? ## The select module is widely known to help with asynchronous I/O on sockets once they are connected. However, it is less than common knowledge how to avoid blocking on the initial connect() call. Jeremy Hylton has the following advice (slightly edited): ## To prevent the TCP connect from blocking, you can set the socket to non-blocking mode. Then when you do the connect(), you will either connect immediately (unlikely) or get an exception that contains the errno. errno.EINPROGRESS indicates that the connection is in progress, but hasn't finished yet. Different OSes will return different errnos, so you're going to have to check. I can tell you that different versions of Solaris return different errno values. ## In Python 1.5 and later, you can use connect_ex() to avoid creating an exception. It will just return the errno value. ## To poll, you can call connect_ex() again later -- 0 or errno.EISCONN indicate that you're connected -- or you can pass this socket to select (checking to see if it is writeable). ## Edit this entry / Log info / Last changed on Tue Feb 24 21:30:45 1998 by GvR ## -------------------------------------------------------------------------------- ## 4.73. How do I specify hexadecimal and octal integers? ## To specify an octal digit, precede the octal value with a zero. For example, to set the variable "a" to the octal value "10" (8 in decimal), type: ## >>> a = 010 ## To verify that this works, you can type "a" and hit enter while in the interpreter, which will cause Python to spit out the current value of "a" in decimal: ## >>> a ## 8 ## Hexadecimal is just as easy. Simply precede the hexadecimal number with a zero, and then a lower or uppercase "x". Hexadecimal digits can be specified in lower or uppercase. For example, in the Python interpreter: ## >>> a = 0xa5 ## >>> a ## 165 ## >>> b = 0XB2 ## >>> b ## 178 ## Edit this entry / Log info / Last changed on Tue Mar 3 12:53:16 1998 by GvR ## -------------------------------------------------------------------------------- ## 4.74. How to get a single keypress at a time? ## For Windows, see question 8.2. Here is an answer for Unix (see also 4.94). ## There are several solutions; some involve using curses, which is a pretty big thing to learn. Here's a solution without curses, due to Andrew Kuchling (adapted from code to do a PGP-style randomness pool): ## import termios, sys, os ## fd = sys.stdin.fileno() ## old = termios.tcgetattr(fd) ## new = termios.tcgetattr(fd) ## new[3] = new[3] & ~termios.ICANON & ~termios.ECHO ## new[6][termios.VMIN] = 1 ## new[6][termios.VTIME] = 0 ## termios.tcsetattr(fd, termios.TCSANOW, new) ## s = '' # We'll save the characters typed and add them to the pool. ## try: ## while 1: ## c = os.read(fd, 1) ## print "Got character", `c` ## s = s+c ## finally: ## termios.tcsetattr(fd, termios.TCSAFLUSH, old) ## You need the termios module for any of this to work, and I've only tried it on Linux, though it should work elsewhere. It turns off stdin's echoing and disables canonical mode, and then reads a character at a time from stdin, noting the time after each keystroke. ## Edit this entry / Log info / Last changed on Thu Oct 24 00:36:56 2002 by chris ## -------------------------------------------------------------------------------- ## 4.75. How can I overload constructors (or methods) in Python? ## (This actually applies to all methods, but somehow the question usually comes up first in the context of constructors.) ## Where in C++ you'd write ## class C { ## C() { cout << "No arguments\n"; } ## C(int i) { cout << "Argument is " << i << "\n"; } ## } ## in Python you have to write a single constructor that catches all cases using default arguments. For example: ## class C: ## def __init__(self, i=None): ## if i is None: ## print "No arguments" ## else: ## print "Argument is", i ## This is not entirely equivalent, but close enough in practice. ## You could also try a variable-length argument list, e.g. ## def __init__(self, *args): ## .... ## The same approach works for all method definitions. ## Edit this entry / Log info / Last changed on Mon Apr 20 11:55:55 1998 by GvR ## -------------------------------------------------------------------------------- ## 4.76. How do I pass keyword arguments from one method to another? ## Use apply. For example: ## class Account: ## def __init__(self, **kw): ## self.accountType = kw.get('accountType') ## self.balance = kw.get('balance') ## class CheckingAccount(Account): ## def __init__(self, **kw): ## kw['accountType'] = 'checking' ## apply(Account.__init__, (self,), kw) ## myAccount = CheckingAccount(balance=100.00) ## In Python 2.0 you can call it directly using the new ** syntax: ## class CheckingAccount(Account): ## def __init__(self, **kw): ## kw['accountType'] = 'checking' ## Account.__init__(self, **kw) ## or more generally: ## >>> def f(x, *y, **z): ## ... print x,y,z ## ... ## >>> Y = [1,2,3] ## >>> Z = {'foo':3,'bar':None} ## >>> f('hello', *Y, **Z) ## hello (1, 2, 3) {'foo': 3, 'bar': None} ## Edit this entry / Log info / Last changed on Thu Dec 28 13:04:01 2000 by Bjorn Pettersen ## -------------------------------------------------------------------------------- ## 4.77. What module should I use to help with generating HTML? ## Check out HTMLgen written by Robin Friedrich. It's a class library of objects corresponding to all the HTML 3.2 markup tags. It's used when you are writing in Python and wish to synthesize HTML pages for generating a web or for CGI forms, etc. ## It can be found in the FTP contrib area on python.org or on the Starship. Use the search engines there to locate the latest version. ## It might also be useful to consider DocumentTemplate, which offers clear separation between Python code and HTML code. DocumentTemplate is part of the Bobo objects publishing system (http:/www.digicool.com/releases) but can be used independantly of course! ## Edit this entry / Log info / Last changed on Fri Aug 28 09:54:58 1998 by GvR ## -------------------------------------------------------------------------------- ## 4.78. How do I create documentation from doc strings? ## Use gendoc, by Daniel Larson. See ## http://starship.python.net/crew/danilo/ ## It can create HTML from the doc strings in your Python source code. ## Edit this entry / Log info / Last changed on Mon Oct 7 17:15:51 2002 by Phil Rittenhouse ## -------------------------------------------------------------------------------- ## 4.79. How do I read (or write) binary data? ## For complex data formats, it's best to use use the struct module. It's documented in the library reference. It allows you to take a string read from a file containing binary data (usually numbers) and convert it to Python objects; and vice versa. ## For example, the following code reads two 2-byte integers and one 4-byte integer in big-endian format from a file: ## import struct ## f = open(filename, "rb") # Open in binary mode for portability ## s = f.read(8) ## x, y, z = struct.unpack(">hhl", s) ## The '>' in the format string forces bin-endian data; the letter 'h' reads one "short integer" (2 bytes), and 'l' reads one "long integer" (4 bytes) from the string. ## For data that is more regular (e.g. a homogeneous list of ints or floats), you can also use the array module, also documented in the library reference. ## Edit this entry / Log info / Last changed on Wed Oct 7 09:16:45 1998 by GvR ## -------------------------------------------------------------------------------- ## 4.80. I can't get key bindings to work in Tkinter ## An oft-heard complaint is that event handlers bound to events with the bind() method don't get handled even when the appropriate key is pressed. ## The most common cause is that the widget to which the binding applies doesn't have "keyboard focus". Check out the Tk documentation for the focus command. Usually a widget is given the keyboard focus by clicking in it (but not for labels; see the taketocus option). ## Edit this entry / Log info / Last changed on Fri Jun 12 09:37:33 1998 by GvR ## -------------------------------------------------------------------------------- ## 4.81. "import crypt" fails ## [Unix] ## Starting with Python 1.5, the crypt module is disabled by default. In order to enable it, you must go into the Python source tree and edit the file Modules/Setup to enable it (remove a '#' sign in front of the line starting with '#crypt'). Then rebuild. You may also have to add the string '-lcrypt' to that same line. ## Edit this entry / Log info / Last changed on Wed Aug 5 08:57:09 1998 by GvR ## -------------------------------------------------------------------------------- ## 4.82. Are there coding standards or a style guide for Python programs? ## Yes, Guido has written the "Python Style Guide". See http://www.python.org/doc/essays/styleguide.html ## Edit this entry / Log info / Last changed on Tue Sep 29 09:50:27 1998 by Joseph VanAndel ## -------------------------------------------------------------------------------- ## 4.83. How do I freeze Tkinter applications? ## Freeze is a tool to create stand-alone applications (see 4.28). ## When freezing Tkinter applications, the applications will not be truly stand-alone, as the application will still need the tcl and tk libraries. ## One solution is to ship the application with the tcl and tk libraries, and point to them at run-time using the TCL_LIBRARY and TK_LIBRARY environment variables. ## To get truly stand-alone applications, the Tcl scripts that form the library have to be integrated into the application as well. One tool supporting that is SAM (stand-alone modules), which is part of the Tix distribution (http://tix.mne.com). Build Tix with SAM enabled, perform the appropriate call to Tclsam_init etc inside Python's Modules/tkappinit.c, and link with libtclsam and libtksam (you might include the Tix libraries as well). ## -------------------------------------------------------------------------------- ## 4.84. How do I create static class data and static class methods? ## [Tim Peters, tim_one@email.msn.com] ## Static data (in the sense of C++ or Java) is easy; static methods (again in the sense of C++ or Java) are not supported directly. ## STATIC DATA ## For example, ## class C: ## count = 0 # number of times C.__init__ called ## def __init__(self): ## C.count = C.count + 1 ## def getcount(self): ## return C.count # or return self.count ## c.count also refers to C.count for any c such that isinstance(c, C) holds, unless overridden by c itself or by some class on the base-class search path from c.__class__ back to C. ## Caution: within a method of C, ## self.count = 42 ## creates a new and unrelated instance vrbl named "count" in self's own dict. So rebinding of a class-static data name needs the ## C.count = 314 ## form whether inside a method or not. ## STATIC METHODS ## Static methods (as opposed to static data) are unnatural in Python, because ## C.getcount ## returns an unbound method object, which can't be invoked without supplying an instance of C as the first argument. ## The intended way to get the effect of a static method is via a module-level function: ## def getcount(): ## return C.count ## If your code is structured so as to define one class (or tightly related class hierarchy) per module, this supplies the desired encapsulation. ## Several tortured schemes for faking static methods can be found by searching DejaNews. Most people feel such cures are worse than the disease. Perhaps the least obnoxious is due to Pekka Pessi (mailto:ppessi@hut.fi): ## # helper class to disguise function objects ## class _static: ## def __init__(self, f): ## self.__call__ = f ## class C: ## count = 0 ## def __init__(self): ## C.count = C.count + 1 ## def getcount(): ## return C.count ## getcount = _static(getcount) ## def sum(x, y): ## return x + y ## sum = _static(sum) ## C(); C() ## c = C() ## print C.getcount() # prints 3 ## print c.getcount() # prints 3 ## print C.sum(27, 15) # prints 42 ## Edit this entry / Log info / Last changed on Thu Jan 21 21:35:38 1999 by Tim Peters ## -------------------------------------------------------------------------------- ## 4.85. __import__('x.y.z') returns ; how do I get z? ## Try ## __import__('x.y.z').y.z ## For more realistic situations, you may have to do something like ## m = __import__(s) ## for i in string.split(s, ".")[1:]: ## m = getattr(m, i) ## Edit this entry / Log info / Last changed on Thu Jan 28 11:01:43 1999 by GvR ## -------------------------------------------------------------------------------- ## 4.86. Basic thread wisdom ## Please note that as of Python 2.1 there is no way to take advantage of multiprocessor hardware using the Python thread model. The interpreter uses a global lock, which does not allow multiple threads to be concurrently active. ## If you write a simple test program like this: ## import thread ## def run(name, n): ## for i in range(n): print name, i ## for i in range(10): ## thread.start_new(run, (i, 100)) ## none of the threads seem to run! The reason is that as soon as the main thread exits, all threads are killed. ## A simple fix is to add a sleep to the end of the program, sufficiently long for all threads to finish: ## import thread, time ## def run(name, n): ## for i in range(n): print name, i ## for i in range(10): ## thread.start_new(run, (i, 100)) ## time.sleep(10) # <----------------------------! ## But now (on many platforms) the threads don't run in parallel, but appear to run sequentially, one at a time! The reason is that the OS thread scheduler doesn't start a new thread until the previous thread is blocked. ## A simple fix is to add a tiny sleep to the start of the run function: ## import thread, time ## def run(name, n): ## time.sleep(0.001) # <---------------------! ## for i in range(n): print name, i ## for i in range(10): ## thread.start_new(run, (i, 100)) ## time.sleep(10) ## Some more hints: ## Instead of using a time.sleep() call at the end, it's better to use some kind of semaphore mechanism. One idea is to use a the Queue module to create a queue object, let each thread append a token to the queue when it finishes, and let the main thread read as many tokens from the queue as there are threads. ## Use the threading module instead of the thread module. It's part of Python since version 1.5.1. It takes care of all these details, and has many other nice features too! ## Edit this entry / Log info / Last changed on Fri May 4 07:51:58 2001 by Steve Holden ## -------------------------------------------------------------------------------- ## 4.87. Why doesn't closing sys.stdout (stdin, stderr) really close it? ## Python file objects are a high-level layer of abstraction on top of C streams, which in turn are a medium-level layer of abstraction on top of (among other things) low-level C file descriptors. ## For most file objects f you create in Python via the builtin "open" function, f.close() marks the Python file object as being closed from Python's point of view, and also arranges to close the underlying C stream. This happens automatically too, in f's destructor, when f becomes garbage. ## But stdin, stdout and stderr are treated specially by Python, because of the special status also given to them by C: doing ## sys.stdout.close() # ditto for stdin and stderr ## marks the Python-level file object as being closed, but does not close the associated C stream (provided sys.stdout is still bound to its default value, which is the stream C also calls "stdout"). ## To close the underlying C stream for one of these three, you should first be sure that's what you really want to do (e.g., you may confuse the heck out of extension modules trying to do I/O). If it is, use os.close: ## os.close(0) # close C's stdin stream ## os.close(1) # close C's stdout stream ## os.close(2) # close C's stderr stream ## Edit this entry / Log info / Last changed on Sat Apr 17 02:22:35 1999 by Tim Peters ## -------------------------------------------------------------------------------- ## 4.88. What kinds of global value mutation are thread-safe? ## [adapted from c.l.py responses by Gordon McMillan & GvR] ## A global interpreter lock is used internally to ensure that only one thread runs in the Python VM at a time. In general, Python offers to switch among threads only between bytecode instructions (how frequently it offers to switch can be set via sys.setcheckinterval). Each bytecode instruction-- and all the C implementation code reached from it --is therefore atomic. ## In theory, this means an exact accounting requires an exact understanding of the PVM bytecode implementation. In practice, it means that operations on shared vrbls of builtin data types (ints, lists, dicts, etc) that "look atomic" really are. ## For example, these are atomic (L, L1, L2 are lists, D, D1, D2 are dicts, x, y are objects, i, j are ints): ## L.append(x) ## L1.extend(L2) ## x = L[i] ## x = L.pop() ## L1[i:j] = L2 ## L.sort() ## x = y ## x.field = y ## D[x] = y ## D1.update(D2) ## D.keys() ## These aren't: ## i = i+1 ## L.append(L[-1]) ## L[i] = L[j] ## D[x] = D[x] + 1 ## Note: operations that replace other objects may invoke those other objects' __del__ method when their reference count reaches zero, and that can affect things. This is especially true for the mass updates to dictionaries and lists. When in doubt, use a mutex! ## Edit this entry / Log info / Last changed on Sun May 2 19:14:07 1999 by Tim Peters ## -------------------------------------------------------------------------------- ## 4.89. How do I modify a string in place? ## Strings are immutable (see question 6.2) so you cannot modify a string directly. If you need an object with this ability, try converting the string to a list or take a look at the array module. ## >>> s = "Hello, world" ## >>> a = list(s) ## >>> print a ## ['H', 'e', 'l', 'l', 'o', ',', ' ', 'w', 'o', 'r', 'l', 'd'] ## >>> a[7:] = list("there!") ## >>> import string ## >>> print string.join(a, '') ## 'Hello, there!' ## >>> import array ## >>> a = array.array('c', s) ## >>> print a ## array('c', 'Hello, world') ## >>> a[0] = 'y' ; print a ## array('c', 'yello world') ## >>> a.tostring() ## 'yello, world' ## Edit this entry / Log info / Last changed on Tue May 18 01:22:47 1999 by Andrew Dalke ## -------------------------------------------------------------------------------- ## 4.90. How to pass on keyword/optional parameters/arguments ## Q: How can I pass on optional or keyword parameters from one function to another? ## def f1(a, *b, **c): ## ... ## A: In Python 2.0 and above: ## def f2(x, *y, **z): ## ... ## z['width']='14.3c' ## ... ## f1(x, *y, **z) ## Note: y can be any sequence (e.g., list or tuple) and z must be a dict. ## A: For versions prior to 2.0, use 'apply', like: ## def f2(x, *y, **z): ## ... ## z['width']='14.3c' ## ... ## apply(f1, (x,)+y, z) ## Edit this entry / Log info / Last changed on Mon Jun 3 07:20:56 2002 by Matthias Urlichs ## -------------------------------------------------------------------------------- ## 4.91. How can I get a dictionary to display its keys in a consistent order? ## In general, dictionaries store their keys in an unpredictable order, so the display order of a dictionary's elements will be similarly unpredictable. (See Question 6.12 to understand why this is so.) ## This can be frustrating if you want to save a printable version to a file, make some changes and then compare it with some other printed dictionary. If you have such needs you can subclass UserDict.UserDict to create a SortedDict class that prints itself in a predictable order. Here's one simpleminded implementation of such a class: ## import UserDict, string ## class SortedDict(UserDict.UserDict): ## def __repr__(self): ## result = [] ## append = result.append ## keys = self.data.keys() ## keys.sort() ## for k in keys: ## append("%s: %s" % (`k`, `self.data[k]`)) ## return "{%s}" % string.join(result, ", ") ## ___str__ = __repr__ ## This will work for many common situations you might encounter, though it's far from a perfect solution. (It won't have any effect on the pprint module and does not transparently handle values that are or contain dictionaries. ## Edit this entry / Log info / Last changed on Thu Sep 16 17:31:06 1999 by Skip Montanaro ## -------------------------------------------------------------------------------- ## 4.92. Is there a Python tutorial? ## Yes. See question 1.20 at http://www.python.org/doc/FAQ.html#1.20 ## Edit this entry / Log info / Last changed on Sat Dec 4 16:04:00 1999 by TAB ## -------------------------------------------------------------------------------- ## 4.93. Deleted ## See 4.28 ## Edit this entry / Log info / Last changed on Tue May 28 20:40:37 2002 by GvR ## -------------------------------------------------------------------------------- ## 4.94. How do I get a single keypress without blocking? ## There are several solutions; some involve using curses, which is a pretty big thing to learn. Here's a solution without curses. (see also 4.74, for Windows, see question 8.2) ## import termios, fcntl, sys, os ## fd = sys.stdin.fileno() ## oldterm = termios.tcgetattr(fd) ## newattr = termios.tcgetattr(fd) ## newattr[3] = newattr[3] & ~termios.ICANON & ~termios.ECHO ## termios.tcsetattr(fd, termios.TCSANOW, newattr) ## oldflags = fcntl.fcntl(fd, fcntl.F_GETFL) ## fcntl.fcntl(fd, fcntl.F_SETFL, oldflags | os.O_NONBLOCK) ## try: ## while 1: ## try: ## c = sys.stdin.read(1) ## print "Got character", `c` ## except IOError: pass ## finally: ## termios.tcsetattr(fd, termios.TCSAFLUSH, oldterm) ## fcntl.fcntl(fd, fcntl.F_SETFL, oldflags) ## You need the termios and the fcntl module for any of this to work, and I've only tried it on Linux, though it should work elsewhere. ## In this code, characters are read and printed one at a time. ## termios.tcsetattr() turns off stdin's echoing and disables canonical mode. fcntl.fnctl() is used to obtain stdin's file descriptor flags and modify them for non-blocking mode. Since reading stdin when it is empty results in an IOError, this error is caught and ignored. ## Edit this entry / Log info / Last changed on Thu Oct 24 00:39:06 2002 by chris ## -------------------------------------------------------------------------------- ## 4.95. Is there an equivalent to Perl chomp()? (Remove trailing newline from string) ## There are two partial substitutes. If you want to remove all trailing whitespace, use the method string.rstrip(). Otherwise, if there is only one line in the string, use string.splitlines()[0]. ## ----------------------------------------------------------------------- ## rstrip() is too greedy, it strips all trailing white spaces. ## splitlines() takes ControlM as line boundary. ## Consider these strings as input: ## "python python \r\n" ## "python\rpython\r\n" ## "python python \r\r\r\n" ## The results from rstrip()/splitlines() are perhaps not what we want. ## It seems re can perform this task. ## #!/usr/bin/python ## # requires python2 ## import re, os, StringIO ## lines=StringIO.StringIO( ## "The Python Programming Language\r\n" ## "The Python Programming Language \r \r \r\r\n" ## "The\rProgramming\rLanguage\r\n" ## "The\rProgramming\rLanguage\r\r\r\r\n" ## "The\r\rProgramming\r\rLanguage\r\r\r\r\n" ## ) ## ln=re.compile("(?:[\r]?\n|\r)$") # dos:\r\n, unix:\n, mac:\r, others: unknown ## # os.linesep does not work if someone ftps(in binary mode) a dos/mac text file ## # to your unix box ## #ln=re.compile(os.linesep + "$") ## while 1: ## s=lines.readline() ## if not s: break ## print "1.(%s)" % `s.rstrip()` ## print "2.(%s)" % `ln.sub( "", s, 1)` ## print "3.(%s)" % `s.splitlines()[0]` ## print "4.(%s)" % `s.splitlines()` ## print ## lines.close() ## Edit this entry / Log info / Last changed on Wed Aug 8 09:51:34 2001 by Crystal ## -------------------------------------------------------------------------------- ## 4.96. Why is join() a string method when I'm really joining the elements of a (list, tuple, sequence)? ## Strings became much more like other standard types starting in release 1.6, when methods were added which give the same functionality that has always been available using the functions of the string module. These new methods have been widely accepted, but the one which appears to make (some) programmers feel uncomfortable is: ## ", ".join(['1', '2', '4', '8', '16']) ## which gives the result ## "1, 2, 4, 8, 16" ## There are two usual arguments against this usage. ## The first runs along the lines of: "It looks really ugly using a method of a string literal (string constant)", to which the answer is that it might, but a string literal is just a fixed value. If the methods are to be allowed on names bound to strings there is no logical reason to make them unavailable on literals. Get over it! ## The second objection is typically cast as: "I am really telling a sequence to join its members together with a string constant". Sadly, you aren't. For some reason there seems to be much less difficulty with having split() as a string method, since in that case it is easy to see that ## "1, 2, 4, 8, 16".split(", ") ## is an instruction to a string literal to return the substrings delimited by the given separator (or, by default, arbitrary runs of white space). In this case a Unicode string returns a list of Unicode strings, an ASCII string returns a list of ASCII strings, and everyone is happy. ## join() is a string method because in using it you are telling the separator string to iterate over an arbitrary sequence, forming string representations of each of the elements, and inserting itself between the elements' representations. This method can be used with any argument which obeys the rules for sequence objects, inluding any new classes you might define yourself. ## Because this is a string method it can work for Unicode strings as well as plain ASCII strings. If join() were a method of the sequence types then the sequence types would have to decide which type of string to return depending on the type of the separator. ## If none of these arguments persuade you, then for the moment you can continue to use the join() function from the string module, which allows you to write ## string.join(['1', '2', '4', '8', '16'], ", ") ## You will just have to try and forget that the string module actually uses the syntax you are compaining about to implement the syntax you prefer! ## Edit this entry / Log info / Last changed on Fri Aug 2 15:51:58 2002 by Steve Holden ## -------------------------------------------------------------------------------- ## 4.97. How can my code discover the name of an object? ## Generally speaking, it can't, because objects don't really have names. The assignment statement does not store the assigned value in the name but a reference to it. Essentially, assignment creates a binding of a name to a value. The same is true of def and class statements, but in that case the value is a callable. Consider the following code: ## class A: ## pass ## B = A ## a = B() ## b = a ## print b ## <__main__.A instance at 016D07CC> ## print a ## <__main__.A instance at 016D07CC> ## Arguably the class has a name: even though it is bound to two names and invoked through the name B the created instance is still reported as an instance of class A. However, it is impossible to say whether the instance's name is a or b, since both names are bound to the same value. ## Generally speaking it should not be necessary for your code to "know the names" of particular values. Unless you are deliberately writing introspective programs, this is usually an indication that a change of approach might be beneficial. ## Edit this entry / Log info / Last changed on Thu Mar 8 03:53:39 2001 by Steve Holden ## -------------------------------------------------------------------------------- ## 4.98. Why are floating point calculations so inaccurate? ## The development version of the Python Tutorial now contains an Appendix with more info: ## http://www.python.org/doc/current/tut/node14.html ## People are often very surprised by results like this: ## >>> 1.2-1.0 ## 0.199999999999999996 ## And think it is a bug in Python. It's not. It's a problem caused by the internal representation of a floating point number. A floating point number is stored as a fixed number of binary digits. ## In decimal math, there are many numbers that can't be represented with a fixed number of decimal digits, i.e. 1/3 = 0.3333333333....... ## In the binary case, 1/2 = 0.1, 1/4 = 0.01, 1/8 = 0.001, etc. There are a lot of numbers that can't be represented. The digits are cut off at some point. ## Since Python 1.6, a floating point's repr() function prints as many digits are necessary to make eval(repr(f)) == f true for any float f. The str() function prints the more sensible number that was probably intended: ## >>> 0.2 ## 0.20000000000000001 ## >>> print 0.2 ## 0.2 ## Again, this has nothing to do with Python, but with the way the underlying C platform handles floating points, and ultimately with the inaccuracy you'll always have when writing down numbers of fixed number of digit strings. ## One of the consequences of this is that it is dangerous to compare the result of some computation to a float with == ! Tiny inaccuracies may mean that == fails. ## Instead try something like this: ## epsilon = 0.0000000000001 # Tiny allowed error ## expected_result = 0.4 ## if expected_result-epsilon <= computation() <= expected_result+epsilon: ## ... ## Edit this entry / Log info / Last changed on Mon Apr 1 22:18:47 2002 by Fred Drake ## -------------------------------------------------------------------------------- ## 4.99. I tried to open Berkeley DB file, but bsddb produces bsddb.error: (22, 'Invalid argument'). Help! How can I restore my data? ## Don't panic! Your data are probably intact. The most frequent cause for the error is that you tried to open an earlier Berkeley DB file with a later version of the Berkeley DB library. ## Many Linux systems now have all three versions of Berkeley DB available. If you are migrating from version 1 to a newer version use db_dump185 to dump a plain text version of the database. If you are migrating from version 2 to version 3 use db2_dump to create a plain text version of the database. In either case, use db_load to create a new native database for the latest version installed on your computer. If you have version 3 of Berkeley DB installed, you should be able to use db2_load to create a native version 2 database. ## You should probably move away from Berkeley DB version 1 files because the hash file code contains known bugs that can corrupt your data. ## Edit this entry / Log info / Last changed on Wed Aug 29 16:04:29 2001 by Skip Montanaro ## -------------------------------------------------------------------------------- ## 4.100. What are the "best practices" for using import in a module? ## First, the standard modules are great. Use them! The standard Python library is large and varied. Using modules can save you time and effort and will reduce maintainenance cost of your code. (Other programs are dedicated to supporting and fixing bugs in the standard Python modules. Coworkers may also be familiar with themodules that you use, reducing the amount of time it takes them to understand your code.) ## The rest of this answer is largely a matter of personal preference, but here's what some newsgroup posters said (thanks to all who responded) ## In general, don't use ## from modulename import * ## Doing so clutters the importer's namespace. Some avoid this idiom even with the few modules that were designed to be imported in this manner. (Modules designed in this manner include Tkinter, thread, and wxPython.) ## Import modules at the top of a file, one module per line. Doing so makes it clear what other modules your code requires and avoids questions of whether the module name is in scope. Using one import per line makes it easy to add and delete module imports. ## Move imports into a local scope (such as at the top of a function definition) if there are a lot of imports, and you're trying to avoid the cost (lots of initialization time) of many imports. This technique is especially helpful if many of the imports are unnecessary depending on how the program executes. You may also want to move imports into a function if the modules are only ever used in that function. Note that loading a module the first time may be expensive (because of the one time initialization of the module) but that loading a module multiple times is virtually free (a couple of dictionary lookups). Even if the module name has gone out of scope, the module is probably available in sys.modules. Thus, there isn't really anything wrong with putting no imports at the module level (if they aren't needed) and putting all of the imports at the function level. ## It is sometimes necessary to move imports to a function or class to avoid problems with circular imports. Gordon says: ## Circular imports are fine where both modules use the "import " ## form of import. They fail when the 2nd module wants to grab a name ## out of the first ("from module import name") and the import is at ## the top level. That's because names in the 1st are not yet available, ## (the first module is busy importing the 2nd). ## In this case, if the 2nd module is only used in one function, then the import can easily be moved into that function. By the time the import is called, the first module will have finished initializing, and the second module can do its import. ## It may also be necessary to move imports out of the top level of code if some of the modules are platform-specific. In that case, it may not even be possible to import all of the modules at the top of the file. In this case, importing the correct modules in the corresponding platform-specific code is a good option. ## If only instances of a specific class uses a module, then it is reasonable to import the module in the class's __init__ method and then assign the module to an instance variable so that the module is always available (via that instance variable) during the life of the object. Note that to delay an import until the class is instantiated, the import must be inside a method. Putting the import inside the class but outside of any method still causes the import to occur when the module is initialized. ## Edit this entry / Log info / Last changed on Sat Aug 4 04:44:47 2001 by TAB ## -------------------------------------------------------------------------------- ## 4.101. Is there a tool to help find bugs or perform static analysis? ## Yes. PyChecker is a static analysis tool for finding bugs in Python source code as well as warning about code complexity and style. ## You can get PyChecker from: http://pychecker.sf.net. ## Edit this entry / Log info / Last changed on Fri Aug 10 15:42:11 2001 by Neal ## -------------------------------------------------------------------------------- ## 4.102. UnicodeError: ASCII [decoding,encoding] error: ordinal not in range(128) ## This error indicates that your Python installation can handle only 7-bit ASCII strings. There are a couple ways to fix or workaround the problem. ## If your programs must handle data in arbitary character set encodings, the environment the application runs in will generally identify the encoding of the data it is handing you. You need to convert the input to Unicode data using that encoding. For instance, a program that handles email or web input will typically find character set encoding information in Content-Type headers. This can then be used to properly convert input data to Unicode. Assuming the string referred to by "value" is encoded as UTF-8: ## value = unicode(value, "utf-8") ## will return a Unicode object. If the data is not correctly encoded as UTF-8, the above call will raise a UnicodeError. ## If you only want strings coverted to Unicode which have non-ASCII data, you can try converting them first assuming an ASCII encoding, and then generate Unicode objects if that fails: ## try: ## x = unicode(value, "ascii") ## except UnicodeError: ## value = unicode(value, "utf-8") ## else: ## # value was valid ASCII data ## pass ## If you normally use a character set encoding other than US-ASCII and only need to handle data in that encoding, the simplest way to fix the problem may be simply to set the encoding in sitecustomize.py. The following code is just a modified version of the encoding setup code from site.py with the relevant lines uncommented. ## # Set the string encoding used by the Unicode implementation. ## # The default is 'ascii' ## encoding = "ascii" # <= CHANGE THIS if you wish ## # Enable to support locale aware default string encodings. ## import locale ## loc = locale.getdefaultlocale() ## if loc[1]: ## encoding = loc[1] ## if encoding != "ascii": ## import sys ## sys.setdefaultencoding(encoding) ## Also note that on Windows, there is an encoding known as "mbcs", which uses an encoding specific to your current locale. In many cases, and particularly when working with COM, this may be an appropriate default encoding to use. ## Edit this entry / Log info / Last changed on Sat Apr 13 04:45:41 2002 by Skip Montanaro ## -------------------------------------------------------------------------------- ## 4.103. Using strings to call functions/methods ## There are various techniques: ## * Use a dictionary pre-loaded with strings and functions. The primary advantage of this technique is that the strings do not need to match the names of the functions. This is also the primary technique used to emulate a case construct: ## def a(): ## pass ## def b(): ## pass ## dispatch = {'go': a, 'stop': b} # Note lack of parens for funcs ## dispatch[get_input()]() # Note trailing parens to call function ## * Use the built-in function getattr(): ## import foo ## getattr(foo, 'bar')() ## Note that getattr() works on any object, including classes, class instances, modules, and so on. ## This is used in several places in the standard library, like this: ## class Foo: ## def do_foo(self): ## ... ## def do_bar(self): ## ... ## f = getattr(foo_instance, 'do_' + opname) ## f() ## * Use locals() or eval() to resolve the function name: ## def myFunc(): ## print "hello" ## fname = "myFunc" ## f = locals()[fname] f() ## f = eval(fname) f() ## Note: Using eval() can be dangerous. If you don't have absolute control over the contents of the string, all sorts of things could happen... ## Edit this entry / Log info / Last changed on Thu Mar 21 08:14:58 2002 by Erno Kuusela ## -------------------------------------------------------------------------------- ## 4.104. How fast are exceptions? ## A try/except block is extremely efficient. Actually executing an exception is expensive. In older versions of Python (prior to 2.0), it was common to code this idiom: ## try: ## value = dict[key] ## except KeyError: ## dict[key] = getinput(key) ## value = dict[key] ## This idiom only made sense when you expected the dict to have the key 95% of the time or more; other times, you coded it like this: ## if dict.has_key(key): ## value = dict[key] ## else: ## dict[key] = getinput(key) ## value = dict[key] ## In Python 2.0 and higher, of course, you code this as ## value = dict.setdefault(key, getinput(key)) ## Edit this entry / Log info / Last changed on Mon Mar 11 03:22:48 2002 by Aahz ## -------------------------------------------------------------------------------- ## 4.105. Sharing global variables across modules ## The canonical way to share information across modules within a single program is to create a special module (often called config or cfg). Just import the config module in all modules of your application; the module then becomes available as a global name. Because there is only one instance of each module, any changes made to the module object get reflected everywhere. For example: ## config.py: ## pass ## mod.py: ## import config ## config.x = 1 ## main.py: ## import config ## import mod ## print config.x ## Note that using a module is also the basis for implementing the Singleton design pattern, for the same reason. ## Edit this entry / Log info / Last changed on Tue Apr 23 23:07:19 2002 by Aahz ## -------------------------------------------------------------------------------- ## 4.106. Why is cPickle so slow? ## Use the binary option. We'd like to make that the default, but it would break backward compatibility: ## largeString = 'z' * (100 * 1024) ## myPickle = cPickle.dumps(largeString, 1) ## Edit this entry / Log info / Last changed on Thu Aug 22 19:54:25 2002 by Aahz ## -------------------------------------------------------------------------------- ## 4.107. When importing module XXX, why do I get "undefined symbol: PyUnicodeUCS2_..." ? ## You are using a version of Python that uses a 4-byte representation for Unicode characters, but the extension module you are importing (possibly indirectly) was compiled using a Python that uses a 2-byte representation for Unicode characters (the default). ## If instead the name of the undefined symbol starts with PyUnicodeUCS4_, the problem is the same by the relationship is reversed: Python was built using 2-byte Unicode characters, and the extension module was compiled using a Python with 4-byte Unicode characters. ## This can easily occur when using pre-built extension packages. RedHat Linux 7.x, in particular, provides a "python2" binary that is compiled with 4-byte Unicode. This only causes the link failure if the extension uses any of the PyUnicode_*() functions. It is also a problem if if an extension uses any of the Unicode-related format specifiers for Py_BuildValue (or similar) or parameter-specifications for PyArg_ParseTuple(). ## You can check the size of the Unicode character a Python interpreter is using by checking the value of sys.maxunicode: ## >>> import sys ## >>> if sys.maxunicode > 65535: ## ... print 'UCS4 build' ## ... else: ## ... print 'UCS2 build' ## The only way to solve this problem is to use extension modules compiled with a Python binary built using the same size for Unicode characters. ## Edit this entry / Log info / Last changed on Tue Aug 27 15:00:17 2002 by Fred Drake ## -------------------------------------------------------------------------------- ## 5. Extending Python = 擴充 Python = ## -------------------------------------------------------------------------------- ## 5.1. Can I create my own functions in C? ## Yes, you can create built-in modules containing functions, variables, exceptions and even new types in C. This is explained in the document "Extending and Embedding the Python Interpreter" (http://www.python.org/doc/current/ext/ext.html). Also read the chapter on dynamic loading. ## There's more information on this in each of the Python books: Programming Python, Internet Programming with Python, and Das Python-Buch (in German). ## Edit this entry / Log info / Last changed on Mon Dec 10 05:18:57 2001 by Fred L. Drake, Jr. ## -------------------------------------------------------------------------------- ## 5.2. Can I create my own functions in C++? ## Yes, using the C-compatibility features found in C++. Basically you place extern "C" { ... } around the Python include files and put extern "C" before each function that is going to be called by the Python interpreter. Global or static C++ objects with constructors are probably not a good idea. ## Edit this entry / Log info ## -------------------------------------------------------------------------------- ## 5.3. How can I execute arbitrary Python statements from C? ## The highest-level function to do this is PyRun_SimpleString() which takes a single string argument which is executed in the context of module __main__ and returns 0 for success and -1 when an exception occurred (including SyntaxError). If you want more control, use PyRun_String(); see the source for PyRun_SimpleString() in Python/pythonrun.c. ## Edit this entry / Log info / Last changed on Fri May 23 20:08:14 1997 by Bill Tutt ## -------------------------------------------------------------------------------- ## 5.4. How can I evaluate an arbitrary Python expression from C? ## Call the function PyRun_String() from the previous question with the start symbol eval_input (Py_eval_input starting with 1.5a1); it parses an expression, evaluates it and returns its value. ## Edit this entry / Log info / Last changed on Wed May 21 22:23:18 1997 by David Ascher ## -------------------------------------------------------------------------------- ## 5.5. How do I extract C values from a Python object? ## That depends on the object's type. If it's a tuple, PyTupleSize(o) returns its length and PyTuple_GetItem(o, i) returns its i'th item; similar for lists with PyListSize(o) and PyList_GetItem(o, i). For strings, PyString_Size(o) returns its length and PyString_AsString(o) a pointer to its value (note that Python strings may contain null bytes so strlen() is not safe). To test which type an object is, first make sure it isn't NULL, and then use PyString_Check(o), PyTuple_Check(o), PyList_Check(o), etc. ## There is also a high-level API to Python objects which is provided by the so-called 'abstract' interface -- read Include/abstract.h for further details. It allows for example interfacing with any kind of Python sequence (e.g. lists and tuples) using calls like PySequence_Length(), PySequence_GetItem(), etc.) as well as many other useful protocols. ## Edit this entry / Log info / Last changed on Wed May 21 22:34:20 1997 by David Ascher ## -------------------------------------------------------------------------------- ## 5.6. How do I use Py_BuildValue() to create a tuple of arbitrary length? ## You can't. Use t = PyTuple_New(n) instead, and fill it with objects using PyTuple_SetItem(t, i, o) -- note that this "eats" a reference count of o. Similar for lists with PyList_New(n) and PyList_SetItem(l, i, o). Note that you must set all the tuple items to some value before you pass the tuple to Python code -- PyTuple_New(n) initializes them to NULL, which isn't a valid Python value. ## Edit this entry / Log info / Last changed on Thu Jul 31 18:15:29 1997 by Guido van Rossum ## -------------------------------------------------------------------------------- ## 5.7. How do I call an object's method from C? ## The PyObject_CallMethod() function can be used to call an arbitrary method of an object. The parameters are the object, the name of the method to call, a format string like that used with Py_BuildValue(), and the argument values: ## PyObject * ## PyObject_CallMethod(PyObject *object, char *method_name, ## char *arg_format, ...); ## This works for any object that has methods -- whether built-in or user-defined. You are responsible for eventually DECREF'ing the return value. ## To call, e.g., a file object's "seek" method with arguments 10, 0 (assuming the file object pointer is "f"): ## res = PyObject_CallMethod(f, "seek", "(ii)", 10, 0); ## if (res == NULL) { ## ... an exception occurred ... ## } ## else { ## Py_DECREF(res); ## } ## Note that since PyObject_CallObject() always wants a tuple for the argument list, to call a function without arguments, pass "()" for the format, and to call a function with one argument, surround the argument in parentheses, e.g. "(i)". ## Edit this entry / Log info / Last changed on Thu Jun 6 16:15:46 2002 by Neal Norwitz ## -------------------------------------------------------------------------------- ## 5.8. How do I catch the output from PyErr_Print() (or anything that prints to stdout/stderr)? ## (Due to Mark Hammond): ## In Python code, define an object that supports the "write()" method. Redirect sys.stdout and sys.stderr to this object. Call print_error, or just allow the standard traceback mechanism to work. Then, the output will go wherever your write() method sends it. ## The easiest way to do this is to use the StringIO class in the standard library. ## Sample code and use for catching stdout: ## >>> class StdoutCatcher: ## ... def __init__(self): ## ... self.data = '' ## ... def write(self, stuff): ## ... self.data = self.data + stuff ## ... ## >>> import sys ## >>> sys.stdout = StdoutCatcher() ## >>> print 'foo' ## >>> print 'hello world!' ## >>> sys.stderr.write(sys.stdout.data) ## foo ## hello world! ## Edit this entry / Log info / Last changed on Wed Dec 16 18:34:25 1998 by Richard Jones ## -------------------------------------------------------------------------------- ## 5.9. How do I access a module written in Python from C? ## You can get a pointer to the module object as follows: ## module = PyImport_ImportModule(""); ## If the module hasn't been imported yet (i.e. it is not yet present in sys.modules), this initializes the module; otherwise it simply returns the value of sys.modules[""]. Note that it doesn't enter the module into any namespace -- it only ensures it has been initialized and is stored in sys.modules. ## You can then access the module's attributes (i.e. any name defined in the module) as follows: ## attr = PyObject_GetAttrString(module, ""); ## Calling PyObject_SetAttrString(), to assign to variables in the module, also works. ## Edit this entry / Log info / Last changed on Wed May 21 22:56:40 1997 by david ascher ## -------------------------------------------------------------------------------- ## 5.10. How do I interface to C++ objects from Python? ## Depending on your requirements, there are many approaches. To do this manually, begin by reading the "Extending and Embedding" document (Doc/ext.tex, see also http://www.python.org/doc/). Realize that for the Python run-time system, there isn't a whole lot of difference between C and C++ -- so the strategy to build a new Python type around a C structure (pointer) type will also work for C++ objects. ## A useful automated approach (which also works for C) is SWIG: http://www.swig.org/. ## Edit this entry / Log info / Last changed on Fri Oct 15 05:14:01 1999 by Sjoerd Mullender ## -------------------------------------------------------------------------------- ## 5.11. mSQLmodule (or other old module) won't build with Python 1.5 (or later) ## Since python-1.4 "Python.h" will have the file includes needed in an extension module. Backward compatibility is dropped after version 1.4 and therefore mSQLmodule.c will not build as "allobjects.h" cannot be found. The following change in mSQLmodule.c is harmless when building it with 1.4 and necessary when doing so for later python versions: ## Remove lines: ## #include "allobjects.h" ## #include "modsupport.h" ## And insert instead: ## #include "Python.h" ## You may also need to add ## #include "rename2.h" ## if the module uses "old names". ## This may happen with other ancient python modules as well, and the same fix applies. ## Edit this entry / Log info / Last changed on Sun Dec 21 02:03:35 1997 by GvR ## -------------------------------------------------------------------------------- ## 5.12. I added a module using the Setup file and the make fails! Huh? ## Setup must end in a newline, if there is no newline there it gets very sad. Aside from this possibility, maybe you have other non-Python-specific linkage problems. ## Edit this entry / Log info / Last changed on Tue Jun 24 15:54:01 1997 by aaron watters ## -------------------------------------------------------------------------------- ## 5.13. I want to compile a Python module on my Red Hat Linux system, but some files are missing. ## Red Hat's RPM for Python doesn't include the /usr/lib/python1.x/config/ directory, which contains various files required for compiling Python extensions. Install the python-devel RPM to get the necessary files. ## Edit this entry / Log info / Last changed on Tue Jan 26 13:44:04 1999 by A.M. Kuchling ## -------------------------------------------------------------------------------- ## 5.14. What does "SystemError: _PyImport_FixupExtension: module yourmodule not loaded" mean? ## This means that you have created an extension module named "yourmodule", but your module init function does not initialize with that name. ## Every module init function will have a line similar to: ## module = Py_InitModule("yourmodule", yourmodule_functions); ## If the string passed to this function is not the same name as your extenion module, the SystemError will be raised. ## Edit this entry / Log info / Last changed on Thu Mar 25 07:16:08 1999 by Mark Hammond ## -------------------------------------------------------------------------------- ## 5.15. How to tell "incomplete input" from "invalid input"? ## Sometimes you want to emulate the Python interactive interpreter's behavior, where it gives you a continuation prompt when the input is incomplete (e.g. you typed the start of an "if" statement or you didn't close your parentheses or triple string quotes), but it gives you a syntax error message immediately when the input is invalid. ## In Python you can use the codeop module, which approximates the parser's behavior sufficiently. IDLE uses this, for example. ## The easiest way to do it in C is to call PyRun_InteractiveLoop() (in a separate thread maybe) and let the Python interpreter handle the input for you. You can also set the PyOS_ReadlineFunctionPointer to point at your custom input function. See Modules/readline.c and Parser/myreadline.c for more hints. ## However sometimes you have to run the embedded Python interpreter in the same thread as your rest application and you can't allow the PyRun_InteractiveLoop() to stop while waiting for user input. The one solution then is to call PyParser_ParseString() and test for e.error equal to E_EOF (then the input is incomplete). Sample code fragment, untested, inspired by code from Alex Farber: ## #include ## #include ## #include ## #include ## #include ## #include ## int testcomplete(char *code) ## /* code should end in \n */ ## /* return -1 for error, 0 for incomplete, 1 for complete */ ## { ## node *n; ## perrdetail e; ## n = PyParser_ParseString(code, &_PyParser_Grammar, ## Py_file_input, &e); ## if (n == NULL) { ## if (e.error == E_EOF) ## return 0; ## return -1; ## } ## PyNode_Free(n); ## return 1; ## } ## Another solution is trying to compile the received string with Py_CompileString(). If it compiles fine - try to execute the returned code object by calling PyEval_EvalCode(). Otherwise save the input for later. If the compilation fails, find out if it's an error or just more input is required - by extracting the message string from the exception tuple and comparing it to the "unexpected EOF while parsing". Here is a complete example using the GNU readline library (you may want to ignore SIGINT while calling readline()): ## #include ## #include ## #include ## #include ## #include ## #include ## int main (int argc, char* argv[]) ## { ## int i, j, done = 0; /* lengths of line, code */ ## char ps1[] = ">>> "; ## char ps2[] = "... "; ## char *prompt = ps1; ## char *msg, *line, *code = NULL; ## PyObject *src, *glb, *loc; ## PyObject *exc, *val, *trb, *obj, *dum; ## Py_Initialize (); ## loc = PyDict_New (); ## glb = PyDict_New (); ## PyDict_SetItemString (glb, "__builtins__", PyEval_GetBuiltins ()); ## while (!done) ## { ## line = readline (prompt); ## if (NULL == line) /* CTRL-D pressed */ ## { ## done = 1; ## } ## else ## { ## i = strlen (line); ## if (i > 0) ## add_history (line); /* save non-empty lines */ ## if (NULL == code) /* nothing in code yet */ ## j = 0; ## else ## j = strlen (code); ## code = realloc (code, i + j + 2); ## if (NULL == code) /* out of memory */ ## exit (1); ## if (0 == j) /* code was empty, so */ ## code[0] = '\0'; /* keep strncat happy */ ## strncat (code, line, i); /* append line to code */ ## code[i + j] = '\n'; /* append '\n' to code */ ## code[i + j + 1] = '\0'; ## src = Py_CompileString (code, "", Py_single_input); ## if (NULL != src) /* compiled just fine - */ ## { ## if (ps1 == prompt || /* ">>> " or */ ## '\n' == code[i + j - 1]) /* "... " and double '\n' */ ## { /* so execute it */ ## dum = PyEval_EvalCode ((PyCodeObject *)src, glb, loc); ## Py_XDECREF (dum); ## Py_XDECREF (src); ## free (code); ## code = NULL; ## if (PyErr_Occurred ()) ## PyErr_Print (); ## prompt = ps1; ## } ## } /* syntax error or E_EOF? */ ## else if (PyErr_ExceptionMatches (PyExc_SyntaxError)) ## { ## PyErr_Fetch (&exc, &val, &trb); /* clears exception! */ ## if (PyArg_ParseTuple (val, "sO", &msg, &obj) && ## !strcmp (msg, "unexpected EOF while parsing")) /* E_EOF */ ## { ## Py_XDECREF (exc); ## Py_XDECREF (val); ## Py_XDECREF (trb); ## prompt = ps2; ## } ## else /* some other syntax error */ ## { ## PyErr_Restore (exc, val, trb); ## PyErr_Print (); ## free (code); ## code = NULL; ## prompt = ps1; ## } ## } ## else /* some non-syntax error */ ## { ## PyErr_Print (); ## free (code); ## code = NULL; ## prompt = ps1; ## } ## free (line); ## } ## } ## Py_XDECREF(glb); ## Py_XDECREF(loc); ## Py_Finalize(); ## exit(0); ## } ## Edit this entry / Log info / Last changed on Wed Mar 15 09:47:24 2000 by Alex Farber ## -------------------------------------------------------------------------------- ## 5.16. How do I debug an extension? ## When using gdb with dynamically loaded extensions, you can't set a breakpoint in your extension until your extension is loaded. ## In your .gdbinit file (or interactively), add the command ## br _PyImport_LoadDynamicModule ## $ gdb /local/bin/python ## gdb) run myscript.py ## gdb) continue # repeat until your extension is loaded ## gdb) finish # so that your extension is loaded ## gdb) br myfunction.c:50 ## gdb) continue ## Edit this entry / Log info / Last changed on Fri Oct 20 11:10:32 2000 by Joe VanAndel ## -------------------------------------------------------------------------------- ## 5.17. How do I find undefined Linux g++ symbols, __builtin_new or __pure_virtural ## To dynamically load g++ extension modules, you must recompile python, relink python using g++ (change LINKCC in the python Modules Makefile), and link your extension module using g++ (e.g., "g++ -shared -o mymodule.so mymodule.o"). ## Edit this entry / Log info / Last changed on Sun Jan 14 18:03:51 2001 by douglas orr ## -------------------------------------------------------------------------------- ## 5.18. How do I define and create objects corresponding to built-in/extension types ## Usually you would like to be able to inherit from a Python type when you ask this question. The bottom line for Python 2.2 is: types and classes are miscible. You build instances by calling classes, and you can build subclasses to your heart's desire. ## You need to be careful when instantiating immutable types like integers or strings. See http://www.amk.ca/python/2.2/, section 2, for details. ## Prior to version 2.2, Python (like Java) insisted that there are first-class and second-class objects (the former are types, the latter classes), and never the twain shall meet. ## The library has, however, done a good job of providing class wrappers for the more commonly desired objects (see UserDict, UserList and UserString for examples), and more are always welcome if you happen to be in the mood to write code. These wrappers still exist in Python 2.2. ## Edit this entry / Log info / Last changed on Mon Jun 10 15:14:07 2002 by Matthias Urlichs ## -------------------------------------------------------------------------------- ## 6. Python's design = Python 的設計理念 = ## -------------------------------------------------------------------------------- ## 6.1. Why isn't there a switch or case statement in Python? == 為什麼沒有 switch 或 case 之類的語句 == ## You can do this easily enough with a sequence of if... elif... elif... else. There have been some proposals for switch statement syntax, but there is no consensus (yet) on whether and how to do range tests. ## Edit this entry / Log info 類似的功能可以用連串的 if... elif... elif.. else 來成. 曾經有人提議過要不要加入這個語句, 但到目前為止尚未達成共識, 問題主要出現在要不要(以及假如要的話,怎樣做)有 range test 的功能。(譯按: 就是像 case 5-34: 或 case Monday - Sunday: 或 case in listx 等.) ## -------------------------------------------------------------------------------- ## 6.2. Why does Python use indentation for grouping of statements? == 為什麼 Python 要用縮排的方式來區分程式碼段 == ## Basically I believe that using indentation for grouping is extremely elegant and contributes a lot to the clarity of the average Python program. Most people learn to love this feature after a while. Some arguments for it: ## Since there are no begin/end brackets there cannot be a disagreement between grouping perceived by the parser and the human reader. I remember long ago seeing a C fragment like this: ## if (x <= y) ## x++; ## y--; ## z++; ## and staring a long time at it wondering why y was being decremented even for x > y... (And I wasn't a C newbie then either.) ## Since there are no begin/end brackets, Python is much less prone to coding-style conflicts. In C there are loads of different ways to place the braces (including the choice whether to place braces around single statements in certain cases, for consistency). If you're used to reading (and writing) code that uses one style, you will feel at least slightly uneasy when reading (or being required to write) another style. Many coding styles place begin/end brackets on a line by themself. This makes programs considerably longer and wastes valuable screen space, making it harder to get a good overview over a program. Ideally, a function should fit on one basic tty screen (say, 20 lines). 20 lines of Python are worth a LOT more than 20 lines of C. This is not solely due to the lack of begin/end brackets (the lack of declarations also helps, and the powerful operations of course), but it certainly helps! ## Edit this entry / Log info / Last changed on Wed May 21 16:00:15 1997 by GvR 作者相信 Python 程式之所以如此簡明易讀很大程度是因為用了這種格式來編程. 很多人經過一段時間後開始喜愛這種優雅的書寫方式. 有個看法指出因為沒有了開始和結尾的標記號，所以用機器和肉眼分析的程式段會是一致的. 參考以下的例子: {{{ if (x <= y) x++; y--; z++; }}} 作者曾瞪了這段程式碼很長的時間但始終轉不過彎來為什麼就算 x > y 但 y 的值還是被減了 1. 自然作者當時已不是 c 的新手了。 另一方面因為沒有了分段的標號, Python 的編寫格式基本上很統一, 不像 c 那樣, 大家對於應該作樣放置那兩個花括號而爭論不休. 就算你已經習慣了某種放法, 但當你看其他寫的和你不一樣的程式時總會覺得有點怪。而且通常慣用的放法是兩個花括號都獨自各佔一行, 如此一來程式會變得很長且浪費了許多寶貴的屏幕空間. 理想的話, 一個函數最好別起過一個終端機的螢幕高度 (比如說是 20 行), 這樣我們才能夠一眼看出個大概。 以 20 行來說, Python 可以完成的工作可比 c 要多好多好多了. 當然, 這不完全是因為 Python 用縮排格式所致 (Python 也需要變量宣告, 而且 Python 有很強的高階運算能力), 但這起碼會有幫助. ## -------------------------------------------------------------------------------- ## 6.3. Why are Python strings immutable? == 為何 Python 的字串被設成不可變量 == ## There are two advantages. One is performance: knowing that a string is immutable makes it easy to lay it out at construction time -- fixed and unchanging storage requirements. (This is also one of the reasons for the distinction between tuples and lists.) The other is that strings in Python are considered as "elemental" as numbers. No amount of activity will change the value 8 to anything else, and in Python, no amount of activity will change the string "eight" to anything else. (Adapted from Jim Roskind) ## Edit this entry / Log info 這樣做有兩個好處. 其一是執行效率: 如果一開始就知道字串是不變的, 在創建時可以更方便一些 -- 因為字串的大小已知且不會改變 (這個原因同樣可以用在 list 和 tuple 的分別上). 另一個原因在於 Python 認為字串是很基本的東西, 和常數是一樣的. 正如你不可能把數字 '8' 改成別的含意, 你也不可能把 "eight" 改成別的意思. (這是根據 Jim Roskind 的說法.) ## -------------------------------------------------------------------------------- ## 6.4. Delete == 已失效 == ## Edit this entry / Log info / Last changed on Tue Jan 2 03:05:25 2001 by Moshe Zadka ## -------------------------------------------------------------------------------- ## 6.5. Why does Python use methods for some functionality (e.g. list.index()) but functions for other (e.g. len(list))? ## The major reason is history. Functions were used for those operations that were generic for a group of types and which were intended to work even for objects that didn't have methods at all (e.g. numbers before type/class unification began, or tuples). ## It is also convenient to have a function that can readily be applied to an amorphous collection of objects when you use the functional features of Python (map(), apply() et al). ## In fact, implementing len(), max(), min() as a built-in function is actually less code than implementing them as methods for each type. One can quibble about individual cases but it's a part of Python, and it's too late to change such things fundamentally now. The functions have to remain to avoid massive code breakage. ## Note that for string operations Python has moved from external functions (the string module) to methods. However, len() is still a function. ## Edit this entry / Log info / Last changed on Thu May 30 14:08:58 2002 by Steve Holden ## -------------------------------------------------------------------------------- ## 6.6. Why can't I derive a class from built-in types (e.g. lists or files)? ## As of Python 2.2, you can derive from built-in types. For previous versions, the answer is: ## This is caused by the relatively late addition of (user-defined) classes to the language -- the implementation framework doesn't easily allow it. See the answer to question 4.2 for a work-around. This may be fixed in the (distant) future. ## Edit this entry / Log info / Last changed on Thu May 23 02:53:22 2002 by Neal Norwitz ## -------------------------------------------------------------------------------- ## 6.7. Why must 'self' be declared and used explicitly in method definitions and calls? ## So, is your current programming language C++ or Java? :-) When classes were added to Python, this was (again) the simplest way of implementing methods without too many changes to the interpreter. The idea was borrowed from Modula-3. It turns out to be very useful, for a variety of reasons. ## First, it makes it more obvious that you are using a method or instance attribute instead of a local variable. Reading "self.x" or "self.meth()" makes it absolutely clear that an instance variable or method is used even if you don't know the class definition by heart. In C++, you can sort of tell by the lack of a local variable declaration (assuming globals are rare or easily recognizable) -- but in Python, there are no local variable declarations, so you'd have to look up the class definition to be sure. ## Second, it means that no special syntax is necessary if you want to explicitly reference or call the method from a particular class. In C++, if you want to use a method from base class that is overridden in a derived class, you have to use the :: operator -- in Python you can write baseclass.methodname(self, ). This is particularly useful for __init__() methods, and in general in cases where a derived class method wants to extend the base class method of the same name and thus has to call the base class method somehow. ## Lastly, for instance variables, it solves a syntactic problem with assignment: since local variables in Python are (by definition!) those variables to which a value assigned in a function body (and that aren't explicitly declared global), there has to be some way to tell the interpreter that an assignment was meant to assign to an instance variable instead of to a local variable, and it should preferably be syntactic (for efficiency reasons). C++ does this through declarations, but Python doesn't have declarations and it would be a pity having to introduce them just for this purpose. Using the explicit "self.var" solves this nicely. Similarly, for using instance variables, having to write "self.var" means that references to unqualified names inside a method don't have to search the instance's directories. ## Edit this entry / Log info / Last changed on Fri Jan 12 08:01:50 2001 by Steve Holden ## -------------------------------------------------------------------------------- ## 6.8. Can't you emulate threads in the interpreter instead of relying on an OS-specific thread implementation? ## Answer 1: Unfortunately, the interpreter pushes at least one C stack frame for each Python stack frame. Also, extensions can call back into Python at almost random moments. Therefore a complete threads implementation requires thread support for C. ## Answer 2: Fortunately, there is Stackless Python, which has a completely redesigned interpreter loop that avoids the C stack. It's still experimental but looks very promising. Although it is binary compatible with standard Python, it's still unclear whether Stackless will make it into the core -- maybe it's just too revolutionary. Stackless Python currently lives here: http://www.stackless.com. A microthread implementation that uses it can be found here: http://world.std.com/~wware/uthread.html. ## Edit this entry / Log info / Last changed on Sat Apr 15 08:18:16 2000 by Just van Rossum ## -------------------------------------------------------------------------------- ## 6.9. Why can't lambda forms contain statements? ## Python lambda forms cannot contain statements because Python's syntactic framework can't handle statements nested inside expressions. ## However, in Python, this is not a serious problem. Unlike lambda forms in other languages, where they add functionality, Python lambdas are only a shorthand notation if you're too lazy to define a function. ## Functions are already first class objects in Python, and can be declared in a local scope. Therefore the only advantage of using a lambda form instead of a locally-defined function is that you don't need to invent a name for the function -- but that's just a local variable to which the function object (which is exactly the same type of object that a lambda form yields) is assigned! ## Edit this entry / Log info / Last changed on Sun Jun 14 14:15:17 1998 by Tim Peters ## -------------------------------------------------------------------------------- ## 6.10. [deleted] ## [lambda vs non-nested scopes used to be here] ## Edit this entry / Log info / Last changed on Thu Mar 21 05:20:56 2002 by Erno Kuusela ## -------------------------------------------------------------------------------- ## 6.11. [deleted] ## [recursive functions vs non-nested scopes used to be here] ## Edit this entry / Log info / Last changed on Thu Mar 21 05:22:04 2002 by Erno Kuusela ## -------------------------------------------------------------------------------- ## 6.12. Why is there no more efficient way of iterating over a dictionary than first constructing the list of keys()? ## As of Python 2.2, you can now iterate over a dictionary directly, using the new implied dictionary iterator: ## for k in d: ... ## There are also methods returning iterators over the values and items: ## for k in d.iterkeys(): # same as above ## for v in d.itervalues(): # iterate over values ## for k, v in d.iteritems(): # iterate over items ## All these require that you do not modify the dictionary during the loop. ## For previous Python versions, the following defense should do: ## Have you tried it? I bet it's fast enough for your purposes! In most cases such a list takes only a few percent of the space occupied by the dictionary. Apart from the fixed header, the list needs only 4 bytes (the size of a pointer) per key. A dictionary uses 12 bytes per key plus between 30 and 70 percent hash table overhead, plus the space for the keys and values. By necessity, all keys are distinct objects, and a string object (the most common key type) costs at least 20 bytes plus the length of the string. Add to that the values contained in the dictionary, and you see that 4 bytes more per item really isn't that much more memory... ## A call to dict.keys() makes one fast scan over the dictionary (internally, the iteration function does exist) copying the pointers to the key objects into a pre-allocated list object of the right size. The iteration time isn't lost (since you'll have to iterate anyway -- unless in the majority of cases your loop terminates very prematurely (which I doubt since you're getting the keys in random order). ## I don't expose the dictionary iteration operation to Python programmers because the dictionary shouldn't be modified during the entire iteration -- if it is, there's a small chance that the dictionary is reorganized because the hash table becomes too full, and then the iteration may miss some items and see others twice. Exactly because this only occurs rarely, it would lead to hidden bugs in programs: it's easy never to have it happen during test runs if you only insert or delete a few items per iteration -- but your users will surely hit upon it sooner or later. ## Edit this entry / Log info / Last changed on Fri May 24 21:24:08 2002 by GvR ## -------------------------------------------------------------------------------- ## 6.13. Can Python be compiled to machine code, C or some other language? ## Not easily. Python's high level data types, dynamic typing of objects and run-time invocation of the interpreter (using eval() or exec) together mean that a "compiled" Python program would probably consist mostly of calls into the Python run-time system, even for seemingly simple operations like "x+1". ## Several projects described in the Python newsgroup or at past Python conferences have shown that this approach is feasible, although the speedups reached so far are only modest (e.g. 2x). JPython uses the same strategy for compiling to Java bytecode. (Jim Hugunin has demonstrated that in combination with whole-program analysis, speedups of 1000x are feasible for small demo programs. See the website for the 1997 Python conference.) ## Internally, Python source code is always translated into a "virtual machine code" or "byte code" representation before it is interpreted (by the "Python virtual machine" or "bytecode interpreter"). In order to avoid the overhead of parsing and translating modules that rarely change over and over again, this byte code is written on a file whose name ends in ".pyc" whenever a module is parsed (from a file whose name ends in ".py"). When the corresponding .py file is changed, it is parsed and translated again and the .pyc file is rewritten. ## There is no performance difference once the .pyc file has been loaded (the bytecode read from the .pyc file is exactly the same as the bytecode created by direct translation). The only difference is that loading code from a .pyc file is faster than parsing and translating a .py file, so the presence of precompiled .pyc files will generally improve start-up time of Python scripts. If desired, the Lib/compileall.py module/script can be used to force creation of valid .pyc files for a given set of modules. ## Note that the main script executed by Python, even if its filename ends in .py, is not compiled to a .pyc file. It is compiled to bytecode, but the bytecode is not saved to a file. ## If you are looking for a way to translate Python programs in order to distribute them in binary form, without the need to distribute the interpreter and library as well, have a look at the freeze.py script in the Tools/freeze directory. This creates a single binary file incorporating your program, the Python interpreter, and those parts of the Python library that are needed by your program. Of course, the resulting binary will only run on the same type of platform as that used to create it. ## Newsflash: there are now several programs that do this, to some extent. Look for Psyco, Pyrex, PyInline, Py2Cmod, and Weave. ## Edit this entry / Log info / Last changed on Fri May 24 21:26:19 2002 by GvR ## -------------------------------------------------------------------------------- ## 6.14. How does Python manage memory? ## The details of Python memory management depend on the implementation. The standard Python implementation (the C implementation) uses reference counting and another mechanism to collect reference cycles. ## Jython relies on the Java runtime; so it uses the JVM's garbage collector. This difference can cause some subtle porting problems if your Python code depends on the behavior of the reference counting implementation. ## The reference cycle collector was added in CPython 2.0. It periodically executes a cycle detection algorithm which looks for inaccessible cycles and deletes the objects involved. A new gc module provides functions to perform a garbage collection, obtain debugging statistics, and tuning the collector's parameters. ## The detection of cycles can be disabled when Python is compiled, if you can't afford even a tiny speed penalty or suspect that the cycle collection is buggy, by specifying the "--without-cycle-gc" switch when running the configure script. ## Sometimes objects get stuck in "tracebacks" temporarily and hence are not deallocated when you might expect. Clear the tracebacks via ## import sys ## sys.exc_traceback = sys.last_traceback = None ## Tracebacks are used for reporting errors and implementing debuggers and related things. They contain a portion of the program state extracted during the handling of an exception (usually the most recent exception). ## In the absence of circularities and modulo tracebacks, Python programs need not explicitly manage memory. ## Why python doesn't use a more traditional garbage collection scheme? For one thing, unless this were added to C as a standard feature, it's a portability pain in the ass. And yes, I know about the Xerox library. It has bits of assembler code for most common platforms. Not for all. And although it is mostly transparent, it isn't completely transparent (when I once linked Python with it, it dumped core). ## Traditional GC also becomes a problem when Python gets embedded into other applications. While in a stand-alone Python it may be fine to replace the standard malloc() and free() with versions provided by the GC library, an application embedding Python may want to have its own substitute for malloc() and free(), and may not want Python's. Right now, Python works with anything that implements malloc() and free() properly. ## In Jython, the following code (which is fine in C Python) will probably run out of file descriptors long before it runs out of memory: ## for file in : ## f = open(file) ## c = f.read(1) ## Using the current reference counting and destructor scheme, each new assignment to f closes the previous file. Using GC, this is not guaranteed. Sure, you can think of ways to fix this. But it's not off-the-shelf technology. If you want to write code that will work with any Python implementation, you should explicitly close the file; this will work regardless of GC: ## for file in : ## f = open(file) ## c = f.read(1) ## f.close() ## Edit this entry / Log info / Last changed on Thu Mar 21 05:35:38 2002 by Erno Kuusela ## -------------------------------------------------------------------------------- ## 6.15. Why are there separate tuple and list data types? ## This is done so that tuples can be immutable while lists are mutable. ## Immutable tuples are useful in situations where you need to pass a few items to a function and don't want the function to modify the tuple; for example, ## point1 = (120, 140) ## point2 = (200, 300) ## record(point1, point2) ## draw(point1, point2) ## You don't want to have to think about what would happen if record() changed the coordinates -- it can't, because the tuples are immutable. ## On the other hand, when creating large lists dynamically, it is absolutely crucial that they are mutable -- adding elements to a tuple one by one requires using the concatenation operator, which makes it quadratic in time. ## As a general guideline, use tuples like you would use structs in C or records in Pascal, use lists like (variable length) arrays. ## Edit this entry / Log info / Last changed on Fri May 23 15:26:03 1997 by GvR ## -------------------------------------------------------------------------------- ## 6.16. How are lists implemented? ## Despite what a Lisper might think, Python's lists are really variable-length arrays. The implementation uses a contiguous array of references to other objects, and keeps a pointer to this array (as well as its length) in a list head structure. ## This makes indexing a list (a[i]) an operation whose cost is independent of the size of the list or the value of the index. ## When items are appended or inserted, the array of references is resized. Some cleverness is applied to improve the performance of appending items repeatedly; when the array must be grown, some extra space is allocated so the next few times don't require an actual resize. ## Edit this entry / Log info / Last changed on Fri May 23 15:32:24 1997 by GvR ## -------------------------------------------------------------------------------- ## 6.17. How are dictionaries implemented? ## Python's dictionaries are implemented as resizable hash tables. ## Compared to B-trees, this gives better performance for lookup (the most common operation by far) under most circumstances, and the implementation is simpler. ## Edit this entry / Log info / Last changed on Fri May 23 23:51:14 1997 by Vladimir Marangozov ## -------------------------------------------------------------------------------- ## 6.18. Why must dictionary keys be immutable? ## The hash table implementation of dictionaries uses a hash value calculated from the key value to find the key. If the key were a mutable object, its value could change, and thus its hash could change. But since whoever changes the key object can't tell that is incorporated in a dictionary, it can't move the entry around in the dictionary. Then, when you try to look up the same object in the dictionary, it won't be found, since its hash value is different; and if you try to look up the old value, it won't be found either, since the value of the object found in that hash bin differs. ## If you think you need to have a dictionary indexed with a list, try to use a tuple instead. The function tuple(l) creates a tuple with the same entries as the list l. ## Some unacceptable solutions that have been proposed: ## - Hash lists by their address (object ID). This doesn't work because if you construct a new list with the same value it won't be found; e.g., ## d = {[1,2]: '12'} ## print d[[1,2]] ## will raise a KeyError exception because the id of the [1,2] used in the second line differs from that in the first line. In other words, dictionary keys should be compared using '==', not using 'is'. ## - Make a copy when using a list as a key. This doesn't work because the list (being a mutable object) could contain a reference to itself, and then the copying code would run into an infinite loop. ## - Allow lists as keys but tell the user not to modify them. This would allow a class of hard-to-track bugs in programs that I'd rather not see; it invalidates an important invariant of dictionaries (every value in d.keys() is usable as a key of the dictionary). ## - Mark lists as read-only once they are used as a dictionary key. The problem is that it's not just the top-level object that could change its value; you could use a tuple containing a list as a key. Entering anything as a key into a dictionary would require marking all objects reachable from there as read-only -- and again, self-referential objects could cause an infinite loop again (and again and again). ## There is a trick to get around this if you need to, but use it at your own risk: You can wrap a mutable structure inside a class instance which has both a __cmp__ and a __hash__ method. ## class listwrapper: ## def __init__(self, the_list): ## self.the_list = the_list ## def __cmp__(self, other): ## return self.the_list == other.the_list ## def __hash__(self): ## l = self.the_list ## result = 98767 - len(l)*555 ## for i in range(len(l)): ## try: ## result = result + (hash(l[i]) % 9999999) * 1001 + i ## except: ## result = (result % 7777777) + i * 333 ## return result ## Note that the hash computation is complicated by the possibility that some members of the list may be unhashable and also by the possibility of arithmetic overflow. ## You must make sure that the hash value for all such wrapper objects that reside in a dictionary (or other hash based structure), remain fixed while the object is in the dictionary (or other structure). ## Furthermore it must always be the case that if o1 == o2 (ie o1.__cmp__(o2)==0) then hash(o1)==hash(o2) (ie, o1.__hash__() == o2.__hash__()), regardless of whether the object is in a dictionary or not. If you fail to meet these restrictions dictionaries and other hash based structures may misbehave! ## In the case of listwrapper above whenever the wrapper object is in a dictionary the wrapped list must not change to avoid anomalies. Don't do this unless you are prepared to think hard about the requirements and the consequences of not meeting them correctly. You've been warned! ## Edit this entry / Log info / Last changed on Thu Jul 10 10:08:40 1997 by aaron watters ## -------------------------------------------------------------------------------- ## 6.19. How the heck do you make an array in Python? ## ["this", 1, "is", "an", "array"] ## Lists are arrays in the C or Pascal sense of the word (see question 6.16). The array module also provides methods for creating arrays of fixed types with compact representations (but they are slower to index than lists). Also note that the Numerics extensions and others define array-like structures with various characteristics as well. ## To get Lisp-like lists, emulate cons cells ## lisp_list = ("like", ("this", ("example", None) ) ) ## using tuples (or lists, if you want mutability). Here the analogue of lisp car is lisp_list[0] and the analogue of cdr is lisp_list[1]. Only do this if you're sure you really need to (it's usually a lot slower than using Python lists). ## Think of Python lists as mutable heterogeneous arrays of Python objects (say that 10 times fast :) ). ## Edit this entry / Log info / Last changed on Wed Aug 13 07:08:27 1997 by aaron watters ## -------------------------------------------------------------------------------- ## 6.20. Why doesn't list.sort() return the sorted list? ## In situations where performance matters, making a copy of the list just to sort it would be wasteful. Therefore, list.sort() sorts the list in place. In order to remind you of that fact, it does not return the sorted list. This way, you won't be fooled into accidentally overwriting a list when you need a sorted copy but also need to keep the unsorted version around. ## As a result, here's the idiom to iterate over the keys of a dictionary in sorted order: ## keys = dict.keys() ## keys.sort() ## for key in keys: ## ...do whatever with dict[key]... ## Edit this entry / Log info / Last changed on Thu Dec 2 17:01:52 1999 by Fred L. Drake, Jr. ## -------------------------------------------------------------------------------- ## 6.21. How do you specify and enforce an interface spec in Python? ## An interfaces specification for a module as provided by languages such as C++ and java describes the prototypes for the methods and functions of the module. Many feel that compile time enforcement of interface specifications help aid in the construction of large programs. Python does not support interface specifications directly, but many of their advantages can be obtained by an appropriate test discipline for components, which can often be very easily accomplished in Python. There is also a tool, PyChecker, which can be used to find problems due to subclassing. ## A good test suite for a module can at once provide a regression test and serve as a module interface specification (even better since it also gives example usage). Look to many of the standard libraries which often have a "script interpretation" which provides a simple "self test." Even modules which use complex external interfaces can often be tested in isolation using trivial "stub" emulations of the external interface. ## An appropriate testing discipline (if enforced) can help build large complex applications in Python as well as having interface specifications would do (or better). Of course Python allows you to get sloppy and not do it. Also you might want to design your code with an eye to make it easily tested. ## Edit this entry / Log info / Last changed on Thu May 23 03:05:29 2002 by Neal Norwitz ## -------------------------------------------------------------------------------- ## 6.22. Why do all classes have the same type? Why do instances all have the same type? ## The Pythonic use of the word "type" is quite different from common usage in much of the rest of the programming language world. A "type" in Python is a description for an object's operations as implemented in C. All classes have the same operations implemented in C which sometimes "call back" to differing program fragments implemented in Python, and hence all classes have the same type. Similarly at the C level all class instances have the same C implementation, and hence all instances have the same type. ## Remember that in Python usage "type" refers to a C implementation of an object. To distinguish among instances of different classes use Instance.__class__, and also look to 4.47. Sorry for the terminological confusion, but at this point in Python's development nothing can be done! ## Edit this entry / Log info / Last changed on Tue Jul 1 12:35:47 1997 by aaron watters ## -------------------------------------------------------------------------------- ## 6.23. Why isn't all memory freed when Python exits? ## Objects referenced from Python module global name spaces are not always deallocated when Python exits. ## This may happen if there are circular references (see question 4.17). There are also certain bits of memory that are allocated by the C library that are impossible to free (e.g. a tool like Purify will complain about these). ## But in general, Python 1.5 and beyond (in contrast with earlier versions) is quite agressive about cleaning up memory on exit. ## If you want to force Python to delete certain things on deallocation use the sys.exitfunc hook to force those deletions. For example if you are debugging an extension module using a memory analysis tool and you wish to make Python deallocate almost everything you might use an exitfunc like this one: ## import sys ## def my_exitfunc(): ## print "cleaning up" ## import sys ## # do order dependant deletions here ## ... ## # now delete everything else in arbitrary order ## for x in sys.modules.values(): ## d = x.__dict__ ## for name in d.keys(): ## del d[name] ## sys.exitfunc = my_exitfunc ## Other exitfuncs can be less drastic, of course. ## (In fact, this one just does what Python now already does itself; but the example of using sys.exitfunc to force cleanups is still useful.) ## Edit this entry / Log info / Last changed on Tue Sep 29 09:46:26 1998 by GvR ## -------------------------------------------------------------------------------- ## 6.24. Why no class methods or mutable class variables? ## The notation ## instance.attribute(arg1, arg2) ## usually translates to the equivalent of ## Class.attribute(instance, arg1, arg2) ## where Class is a (super)class of instance. Similarly ## instance.attribute = value ## sets an attribute of an instance (overriding any attribute of a class that instance inherits). ## Sometimes programmers want to have different behaviours -- they want a method which does not bind to the instance and a class attribute which changes in place. Python does not preclude these behaviours, but you have to adopt a convention to implement them. One way to accomplish this is to use "list wrappers" and global functions. ## def C_hello(): ## print "hello" ## class C: ## hello = [C_hello] ## counter = [0] ## I = C() ## Here I.hello[0]() acts very much like a "class method" and I.counter[0] = 2 alters C.counter (and doesn't override it). If you don't understand why you'd ever want to do this, that's because you are pure of mind, and you probably never will want to do it! This is dangerous trickery, not recommended when avoidable. (Inspired by Tim Peter's discussion.) ## In Python 2.2, you can do this using the new built-in operations classmethod and staticmethod. See http://www.python.org/2.2/descrintro.html#staticmethods ## Edit this entry / Log info / Last changed on Tue Sep 11 15:59:37 2001 by GvR ## -------------------------------------------------------------------------------- ## 6.25. Why are default values sometimes shared between objects? ## It is often expected that a function CALL creates new objects for default values. This is not what happens. Default values are created when the function is DEFINED, that is, there is only one such object that all functions refer to. If that object is changed, subsequent calls to the function will refer to this changed object. By definition, immutable objects (like numbers, strings, tuples, None) are safe from change. Changes to mutable objects (like dictionaries, lists, class instances) is what causes the confusion. ## Because of this feature it is good programming practice not to use mutable objects as default values, but to introduce them in the function. Don't write: ## def foo(dict={}): # XXX shared reference to one dict for all calls ## ... ## but: ## def foo(dict=None): ## if dict is None: ## dict = {} # create a new dict for local namespace ## See page 182 of "Internet Programming with Python" for one discussion of this feature. Or see the top of page 144 or bottom of page 277 in "Programming Python" for another discussion. ## Edit this entry / Log info / Last changed on Sat Aug 16 07:03:35 1997 by Case Roole ## -------------------------------------------------------------------------------- ## 6.26. Why no goto? ## Actually, you can use exceptions to provide a "structured goto" that even works across function calls. Many feel that exceptions can conveniently emulate all reasonable uses of the "go" or "goto" constructs of C, Fortran, and other languages. For example: ## class label: pass # declare a label ## try: ## ... ## if (condition): raise label() # goto label ## ... ## except label: # where to goto ## pass ## ... ## This doesn't allow you to jump into the middle of a loop, but that's usually considered an abuse of goto anyway. Use sparingly. ## Edit this entry / Log info / Last changed on Wed Sep 10 07:16:44 1997 by aaron watters ## -------------------------------------------------------------------------------- ## 6.27. How do you make a higher order function in Python? ## You have two choices: you can use default arguments and override them or you can use "callable objects." For example suppose you wanted to define linear(a,b) which returns a function f where f(x) computes the value a*x+b. Using default arguments: ## def linear(a,b): ## def result(x, a=a, b=b): ## return a*x + b ## return result ## Or using callable objects: ## class linear: ## def __init__(self, a, b): ## self.a, self.b = a,b ## def __call__(self, x): ## return self.a * x + self.b ## In both cases: ## taxes = linear(0.3,2) ## gives a callable object where taxes(10e6) == 0.3 * 10e6 + 2. ## The defaults strategy has the disadvantage that the default arguments could be accidentally or maliciously overridden. The callable objects approach has the disadvantage that it is a bit slower and a bit longer. Note however that a collection of callables can share their signature via inheritance. EG ## class exponential(linear): ## # __init__ inherited ## def __call__(self, x): ## return self.a * (x ** self.b) ## On comp.lang.python, zenin@bawdycaste.org points out that an object can encapsulate state for several methods in order to emulate the "closure" concept from functional programming languages, for example: ## class counter: ## value = 0 ## def set(self, x): self.value = x ## def up(self): self.value=self.value+1 ## def down(self): self.value=self.value-1 ## count = counter() ## inc, dec, reset = count.up, count.down, count.set ## Here inc, dec and reset act like "functions which share the same closure containing the variable count.value" (if you like that way of thinking). ## Edit this entry / Log info / Last changed on Fri Sep 25 08:38:35 1998 by Aaron Watters ## -------------------------------------------------------------------------------- ## 6.28. Why do I get a SyntaxError for a 'continue' inside a 'try'? ## This is an implementation limitation, caused by the extremely simple-minded way Python generates bytecode. The try block pushes something on the "block stack" which the continue would have to pop off again. The current code generator doesn't have the data structures around so that 'continue' can generate the right code. ## Note that JPython doesn't have this restriction! ## Edit this entry / Log info / Last changed on Fri May 22 15:01:07 1998 by GvR ## -------------------------------------------------------------------------------- ## 6.29. Why can't raw strings (r-strings) end with a backslash? ## More precisely, they can't end with an odd number of backslashes: the unpaired backslash at the end escapes the closing quote character, leaving an unterminated string. ## Raw strings were designed to ease creating input for processors (chiefly regular expression engines) that want to do their own backslash escape processing. Such processors consider an unmatched trailing backslash to be an error anyway, so raw strings disallow that. In return, they allow you to pass on the string quote character by escaping it with a backslash. These rules work well when r-strings are used for their intended purpose. ## If you're trying to build Windows pathnames, note that all Windows system calls accept forward slashes too: ## f = open("/mydir/file.txt") # works fine! ## If you're trying to build a pathname for a DOS command, try e.g. one of ## dir = r"\this\is\my\dos\dir" "\\" ## dir = r"\this\is\my\dos\dir\ "[:-1] ## dir = "\\this\\is\\my\\dos\\dir\\" ## Edit this entry / Log info / Last changed on Mon Jul 13 20:50:20 1998 by Tim Peters ## -------------------------------------------------------------------------------- ## 6.30. Why can't I use an assignment in an expression? ## Many people used to C or Perl complain that they want to be able to use e.g. this C idiom: ## while (line = readline(f)) { ## ...do something with line... ## } ## where in Python you're forced to write this: ## while 1: ## line = f.readline() ## if not line: ## break ## ...do something with line... ## This issue comes up in the Python newsgroup with alarming frequency -- search Deja News for past messages about assignment expression. The reason for not allowing assignment in Python expressions is a common, hard-to-find bug in those other languages, caused by this construct: ## if (x = 0) { ## ...error handling... ## } ## else { ## ...code that only works for nonzero x... ## } ## Many alternatives have been proposed. Most are hacks that save some typing but use arbitrary or cryptic syntax or keywords, and fail the simple criterion that I use for language change proposals: it should intuitively suggest the proper meaning to a human reader who has not yet been introduced with the construct. ## The earliest time something can be done about this will be with Python 2.0 -- if it is decided that it is worth fixing. An interesting phenomenon is that most experienced Python programmers recognize the "while 1" idiom and don't seem to be missing the assignment in expression construct much; it's only the newcomers who express a strong desire to add this to the language. ## One fairly elegant solution would be to introduce a new operator for assignment in expressions spelled ":=" -- this avoids the "=" instead of "==" problem. It would have the same precedence as comparison operators but the parser would flag combination with other comparisons (without disambiguating parentheses) as an error. ## Finally -- there's an alternative way of spelling this that seems attractive but is generally less robust than the "while 1" solution: ## line = f.readline() ## while line: ## ...do something with line... ## line = f.readline() ## The problem with this is that if you change your mind about exactly how you get the next line (e.g. you want to change it into sys.stdin.readline()) you have to remember to change two places in your program -- the second one hidden at the bottom of the loop. ## Edit this entry / Log info / Last changed on Tue May 18 00:57:41 1999 by Andrew Dalke ## -------------------------------------------------------------------------------- ## 6.31. Why doesn't Python have a "with" statement like some other languages? ## Basically, because such a construct would be terribly ambiguous. Thanks to Carlos Ribeiro for the following remarks: ## Some languages, such as Object Pascal, Delphi, and C++, use static types. So it is possible to know, in an unambiguous way, what member is being assigned in a "with" clause. This is the main point - the compiler always knows the scope of every variable at compile time. ## Python uses dynamic types. It is impossible to know in advance which attribute will be referenced at runtime. Member attributes may be added or removed from objects on the fly. This would make it impossible to know, from a simple reading, what attribute is being referenced - a local one, a global one, or a member attribute. ## For instance, take the following snippet (it is incomplete btw, just to give you the idea): ## def with_is_broken(a): ## with a: ## print x ## The snippet assumes that "a" must have a member attribute called "x". However, there is nothing in Python that guarantees that. What should happen if "a" is, let us say, an integer? And if I have a global variable named "x", will it end up being used inside the with block? As you see, the dynamic nature of Python makes such choices much harder. ## The primary benefit of "with" and similar language features (reduction of code volume) can, however, easily be achieved in Python by assignment. Instead of: ## function(args).dict[index][index].a = 21 ## function(args).dict[index][index].b = 42 ## function(args).dict[index][index].c = 63 ## would become: ## ref = function(args).dict[index][index] ## ref.a = 21 ## ref.b = 42 ## ref.c = 63 ## This also has the happy side-effect of increasing execution speed, since name bindings are resolved at run-time in Python, and the second method only needs to perform the resolution once. If the referenced object does not have a, b and c attributes, of course, the end result is still a run-time exception. ## Edit this entry / Log info / Last changed on Fri Jan 11 14:32:58 2002 by Steve Holden ## -------------------------------------------------------------------------------- ## 6.32. Why are colons required for if/while/def/class? ## The colon is required primarily to enhance readability (one of the results of the experimental ABC language). Consider this: ## if a==b ## print a ## versus ## if a==b: ## print a ## Notice how the second one is slightly easier to read. Notice further how a colon sets off the example in the second line of this FAQ answer; it's a standard usage in English. Finally, the colon makes it easier for editors with syntax highlighting. ## Edit this entry / Log info / Last changed on Mon Jun 3 07:22:57 2002 by Matthias Urlichs ## -------------------------------------------------------------------------------- ## 7. Using Python on non-UNIX platforms = 在非 UNIX 平台上使用 Python = ## -------------------------------------------------------------------------------- ## 7.1. Is there a Mac version of Python? == Python 有沒有供 Mac 機用的版本 == ## Yes, it is maintained by Jack Jansen. See Jack's MacPython Page: ## http://www.cwi.nl/~jack/macpython.html ## Edit this entry / Log info / Last changed on Fri May 4 09:33:42 2001 by GvR 有哇. 由 Jack Jansen 所維護. 請上他的網站: －　http://www.cwi.nl/~jack/macpython.html ## -------------------------------------------------------------------------------- ## 7.2. Are there DOS and Windows versions of Python? == 有沒有 DOS 和　Windows 的版本 == ## Yes. The core windows binaries are available from http://www.python.org/windows/. There is a plethora of Windows extensions available, including a large number of not-always-compatible GUI toolkits. The core binaries include the standard Tkinter GUI extension. ## Most windows extensions can be found (or referenced) at http://www.python.org/windows/ ## Windows 3.1/DOS support seems to have dropped off recently. You may need to settle for an old version of Python one these platforms. One such port is WPY ## WPY: Ports to DOS, Windows 3.1(1), Windows 95, Windows NT and OS/2. Also contains a GUI package that offers portability between Windows (not DOS) and Unix, and native look and feel on both. ftp://ftp.python.org/pub/python/wpy/. ## Edit this entry / Log info / Last changed on Tue Jun 2 20:21:57 1998 by Mark Hammond 有. 基本的版本可以在 http://www.python.org/windows/ 上拿到. 在該頁面上還有各類的擴展模組選擇, 包括許多不一定完全兼容的圖形編程介面. 基本發佈版中只包括標準的 Tkinter 模組. 大部分的擴充模組可以在 http://www.python.org/windows/ 上取得從那裏的超連結裏得到. 至於 Windows 3.1/DOS 的支授最近已停止了, 使用這些平台的用戶只能將就用較舊的 Python 版本. 其中一個是 WPY WPY: 把 Python 移植到 DOS, Windows 3.1(1), Windows 95, Windows NT 及 OS/2. 同時提供 GUI 工具兼容於 Windows 和 Unix (DOS 不能用), 地址是 ftp://ftp.python.org/pub/python/wpy/. ## -------------------------------------------------------------------------------- ## 7.3. Is there an OS/2 version of Python? == 有沒有 OS/2 的版本 == ## Yes, see http://www.python.org/download/download_os2.html. ## Edit this entry / Log info / Last changed on Tue Sep 7 11:33:16 1999 by GvR 有的, 請看 http://www.python.org/download/download_os2.html. ## -------------------------------------------------------------------------------- ## 7.4. Is there a VMS version of Python? == 有沒有 VMS 版 == ## Jean-Fran?is Pi?onne has ported 2.1.3 to OpenVMS. It can be found at . ## Edit this entry / Log info / Last changed on Thu Sep 19 15:40:38 2002 by Skip Montanaro 有. Jean-Francis Pi?onne 移植至 OpenVMS. 可在 http://vmspython.dyndns.org 上找到. ## -------------------------------------------------------------------------------- ## 7.5. What about IBM mainframes, or other non-UNIX platforms? == IBM 大型機有沒有, 其他非 Unix 機又如何 == ## I haven't heard about these, except I remember hearing about an OS/9 port and a port to Vxworks (both operating systems for embedded systems). If you're interested in any of this, go directly to the newsgroup and ask there, you may find exactly what you need. For example, a port to MPE/iX 5.0 on HP3000 computers was just announced, see http://www.allegro.com/software/. ## Edit this entry / Log info 作者沒聽過有關消息, 不過記得有 OS/9 和 Vxwork 上的版本 (兩個都是嵌入式系統用的). 如果你想知道有關的發展, 可以試試到新聞組上問一下, 可能直接有答案. 譬如說最近就公布了 HP3000 上的 MPE/iX5.0 就擁有了它的 Python 版本. (http://www.allegro.com/software/). ## -------------------------------------------------------------------------------- ## 7.6. Where are the source or Makefiles for the non-UNIX versions? == 供非 Unix 機用的程式源碼在哪裏有 == ## The standard sources can (almost) be used. Additional sources can be found in the platform-specific subdirectories of the distribution. ## Edit this entry / Log info 所有的碼都包括在標準的源碼發布版中了. 有關特殊平台的程式碼放在和該平台同名的子目錄中. ## -------------------------------------------------------------------------------- ## 7.7. What is the status and support for the non-UNIX versions? ## I don't have access to most of these platforms, so in general I am dependent on material submitted by volunteers. However I strive to integrate all changes needed to get it to compile on a particular platform back into the standard sources, so porting of the next version to the various non-UNIX platforms should be easy. (Note that Linux is classified as a UNIX platform here. :-) ## Some specific platforms: ## Windows: all versions (95, 98, ME, NT, 2000, XP) are supported, all python.org releases come with a Windows installer. ## MacOS: Jack Jansen does an admirable job of keeping the Mac version up to date (both MacOS X and older versions); see http://www.cwi.nl/~jack/macpython.html ## For all supported platforms, see http://www.python.org/download/ (follow the link to "Other platforms" for less common platforms) ## Edit this entry / Log info / Last changed on Fri May 24 21:34:24 2002 by GvR ## -------------------------------------------------------------------------------- ## 7.8. I have a PC version but it appears to be only a binary. Where's the library? ## If you are running any version of Windows, then you have the wrong distribution. The FAQ lists current Windows versions. Notably, Pythonwin and wpy provide fully functional installations. ## But if you are sure you have the only distribution with a hope of working on your system, then... ## You still need to copy the files from the distribution directory "python/Lib" to your system. If you don't have the full distribution, you can get the file lib.tar.gz from most ftp sites carrying Python; this is a subset of the distribution containing just those files, e.g. ftp://ftp.python.org/pub/python/src/lib1.4.tar.gz. ## Once you have installed the library, you need to point sys.path to it. Assuming the library is in C:\misc\python\lib, the following commands will point your Python interpreter to it (note the doubled backslashes -- you can also use single forward slashes instead): ## >>> import sys ## >>> sys.path.insert(0, 'C:\\misc\\python\\lib') ## >>> ## For a more permanent effect, set the environment variable PYTHONPATH, as follows (talking to a DOS prompt): ## C> SET PYTHONPATH=C:\misc\python\lib ## Edit this entry / Log info / Last changed on Fri May 23 16:28:27 1997 by Ken Manheimer ## -------------------------------------------------------------------------------- ## 7.9. Where's the documentation for the Mac or PC version? ## The documentation for the Unix version also applies to the Mac and PC versions. Where applicable, differences are indicated in the text. ## Edit this entry / Log info ## -------------------------------------------------------------------------------- ## 7.10. How do I create a Python program file on the Mac or PC? ## Use an external editor. On the Mac, BBEdit seems to be a popular no-frills text editor. I work like this: start the interpreter; edit a module file using BBedit; import and test it in the interpreter; edit again in BBedit; then use the built-in function reload() to re-read the imported module; etc. In the 1.4 distribution you will find a BBEdit extension that makes life a little easier: it can tell the interpreter to execute the current window. See :Mac:Tools:BBPy:README. ## Regarding the same question for the PC, Kurt Wm. Hemr writes: "While anyone with a pulse could certainly figure out how to do the same on MS-Windows, I would recommend the NotGNU Emacs clone for MS-Windows. Not only can you easily resave and "reload()" from Python after making changes, but since WinNot auto-copies to the clipboard any text you select, you can simply select the entire procedure (function) which you changed in WinNot, switch to QWPython, and shift-ins to reenter the changed program unit." ## If you're using Windows95 or Windows NT, you should also know about PythonWin, which provides a GUI framework, with an mouse-driven editor, an object browser, and a GUI-based debugger. See ## http://www.python.org/ftp/python/pythonwin/ ## for details. ## Edit this entry / Log info / Last changed on Sun May 25 10:04:25 1997 by GvR ## -------------------------------------------------------------------------------- ## 7.11. How can I use Tkinter on Windows 95/NT? ## Starting from Python 1.5, it's very easy -- just download and install Python and Tcl/Tk and you're in business. See ## http://www.python.org/download/download_windows.html ## One warning: don't attempt to use Tkinter from PythonWin (Mark Hammond's IDE). Use it from the command line interface (python.exe) or the windowless interpreter (pythonw.exe). ## Edit this entry / Log info / Last changed on Fri Jun 12 09:32:48 1998 by GvR ## -------------------------------------------------------------------------------- ## 7.12. cgi.py (or other CGI programming) doesn't work sometimes on NT or win95! ## Be sure you have the latest python.exe, that you are using python.exe rather than a GUI version of python and that you have configured the server to execute ## "...\python.exe -u ..." ## for the cgi execution. The -u (unbuffered) option on NT and win95 prevents the interpreter from altering newlines in the standard input and output. Without it post/multipart requests will seem to have the wrong length and binary (eg, GIF) responses may get garbled (resulting in, eg, a "broken image"). ## Edit this entry / Log info / Last changed on Wed Jul 30 10:48:02 1997 by aaron watters ## -------------------------------------------------------------------------------- ## 7.13. Why doesn't os.popen() work in PythonWin on NT? ## The reason that os.popen() doesn't work from within PythonWin is due to a bug in Microsoft's C Runtime Library (CRT). The CRT assumes you have a Win32 console attached to the process. ## You should use the win32pipe module's popen() instead which doesn't depend on having an attached Win32 console. ## Example: ## import win32pipe ## f = win32pipe.popen('dir /c c:\\') ## print f.readlines() ## f.close() ## Edit this entry / Log info / Last changed on Thu Jul 31 15:34:09 1997 by Bill Tutt ## -------------------------------------------------------------------------------- ## 7.14. How do I use different functionality on different platforms with the same program? ## Remember that Python is extremely dynamic and that you can use this dynamism to configure a program at run-time to use available functionality on different platforms. For example you can test the sys.platform and import different modules based on its value. ## import sys ## if sys.platform == "win32": ## import win32pipe ## popen = win32pipe.popen ## else: ## import os ## popen = os.popen ## (See FAQ 7.13 for an explanation of why you might want to do something like this.) Also you can try to import a module and use a fallback if the import fails: ## try: ## import really_fast_implementation ## choice = really_fast_implementation ## except ImportError: ## import slower_implementation ## choice = slower_implementation ## Edit this entry / Log info / Last changed on Wed Aug 13 07:39:06 1997 by aaron watters ## -------------------------------------------------------------------------------- ## 7.15. Is there an Amiga version of Python? ## Yes. See the AmigaPython homepage at http://www.bigfoot.com/~irmen/python.html. ## Edit this entry / Log info / Last changed on Mon Dec 14 06:53:32 1998 by Irmen de Jong ## -------------------------------------------------------------------------------- ## 7.16. Why doesn't os.popen()/win32pipe.popen() work on Win9x? ## There is a bug in Win9x that prevents os.popen/win32pipe.popen* from working. The good news is there is a way to work around this problem. The Microsoft Knowledge Base article that you need to lookup is: Q150956. You will find links to the knowledge base at: http://www.microsoft.com/kb. ## Edit this entry / Log info / Last changed on Fri Jun 25 10:45:38 1999 by Bill Tutt ## -------------------------------------------------------------------------------- ## 8. Python on Windows = Python用在微軟視窗上 = ## -------------------------------------------------------------------------------- ## 8.1. Using Python for CGI on Microsoft Windows == 在 Windows 中使用 Python CGI == ## ** Setting up the Microsoft IIS Server/Peer Server ## On the Microsoft IIS server or on the Win95 MS Personal Web Server you set up python in the same way that you would set up any other scripting engine. ## Run regedt32 and go to: ## HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\W3SVC\Parameters\ScriptMap ## and enter the following line (making any specific changes that your system may need) ## .py :REG_SZ: c:\\python.exe -u %s %s ## This line will allow you to call your script with a simple reference like: http://yourserver/scripts/yourscript.py provided "scripts" is an "executable" directory for your server (which it usually is by default). The "-u" flag specifies unbuffered and binary mode for stdin - needed when working with binary data ## In addition, it is recommended by people who would know that using ".py" may not be a good idea for the file extensions when used in this context (you might want to reserve *.py for support modules and use *.cgi or *.cgp for "main program" scripts). However, that issue is beyond this Windows FAQ entry. ** IIS 服務器/ Peer 服務器 在這兩種服務器上設置 python 的方式和設置其他即譯語言相同. 先執行 'regedt32' 然後修改 HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\W3SVC\Parameters\ScriptMap 成 '.py :REG_SZ: c:\\python.exe -u %s %s' 如此一來 http://yourserver/scripts/yourscripts.py 就可以運行你的 CGI 程式了. 記得其中 'scripts' 目錄要設定成可執行態 (一般的定態已經是可執行的了). 至於 '-u' 的選項則是指明標準輸入為 'unbuffered and binary' 模式 -- 處理二進位數據時要用的. 另外要指出的是使用 '.py' 檔名來為 CGI 命名並不太好, 但這並不在本題所討論的範圍中. (譬如你可以用 '.py' 來特指模組, 而 '.cgi' 或 '.cgp' 來代表 CGI . ## ** Apache configuration ## In the Apache configuration file httpd.conf, add the following line at the end of the file: ## ScriptInterpreterSource Registry ## Then, give your Python CGI-scripts the extension .py and put them in the cgi-bin directory. ## ** Netscape Servers: Information on this topic exists at: http://home.netscape.com/comprod/server_central/support/fasttrack_man/programs.htm#1010870 ## Edit this entry / Log info / Last changed on Wed Mar 27 12:25:54 2002 by Gerhard H?ing ** Apache 設置 在 Apache 的設定文件 httpd.conf 的末端加入 'ScriptInterpreterSource Registry' 就可以了. ** Netscape Servers: 在關資料請到網上 http://home.netscape.com/comprod/server_central/support/fasttrack_man/programs.htm#1010870. ## -------------------------------------------------------------------------------- ## 8.2. How to check for a keypress without blocking? ## Use the msvcrt module. This is a standard Windows-specific extensions in Python 1.5 and beyond. It defines a function kbhit() which checks whether a keyboard hit is present; also getch() which gets one character without echo. Plus a few other goodies. ## (Search for "keypress" to find an answer for Unix as well.) ## Edit this entry / Log info / Last changed on Mon Mar 30 16:21:46 1998 by GvR ## -------------------------------------------------------------------------------- ## 8.3. $PYTHONPATH ## In MS-DOS derived environments, a unix variable such as $PYTHONPATH is set as PYTHONPATH, without the dollar sign. PYTHONPATH is useful for specifying the location of library files. ## Edit this entry / Log info / Last changed on Thu Jun 11 00:41:26 1998 by Gvr ## -------------------------------------------------------------------------------- ## 8.4. dedent syntax errors ## The FAQ does not recommend using tabs, and Guido's Python Style Guide recommends 4 spaces for distributed Python code; this is also the Emacs python-mode default; see ## http://www.python.org/doc/essays/styleguide.html ## Under any editor mixing tabs and spaces is a bad idea. MSVC is no different in this respect, and is easily configured to use spaces: Take Tools -> Options -> Tabs, and for file type "Default" set "Tab size" and "Indent size" to 4, and select the "Insert spaces" radio button. ## If you suspect mixed tabs and spaces are causing problems in leading whitespace, run Python with the -t switch or, run Tools/Scripts/tabnanny.py to check a directory tree in batch mode. ## Edit this entry / Log info / Last changed on Mon Feb 12 15:04:14 2001 by Steve Holden ## -------------------------------------------------------------------------------- ## 8.5. How do I emulate os.kill() in Windows? ## Use win32api: ## def kill(pid): ## """kill function for Win32""" ## import win32api ## handle = win32api.OpenProcess(1, 0, pid) ## return (0 != win32api.TerminateProcess(handle, 0)) ## Edit this entry / Log info / Last changed on Sat Aug 8 18:55:06 1998 by Jeff Bauer ## -------------------------------------------------------------------------------- ## 8.6. Why does os.path.isdir() fail on NT shared directories? ## The solution appears to be always append the "\\" on the end of shared drives. ## >>> import os ## >>> os.path.isdir( '\\\\rorschach\\public') ## 0 ## >>> os.path.isdir( '\\\\rorschach\\public\\') ## 1 ## [Blake Winton responds:] I've had the same problem doing "Start >> Run" and then a directory on a shared drive. If I use "\\rorschach\public", it will fail, but if I use "\\rorschach\public\", it will work. For that matter, os.stat() does the same thing (well, it gives an error for "\\\\rorschach\\public", but you get the idea)... ## I've got a theory about why this happens, but it's only a theory. NT knows the difference between shared directories, and regular directories. "\\rorschach\public" isn't a directory, it's _really_ an IPC abstraction. This is sort of lended credence to by the fact that when you're mapping a network drive, you can't map "\\rorschach\public\utils", but only "\\rorschach\public". ## [Clarification by funkster@midwinter.com] It's not actually a Python question, as Python is working just fine; it's clearing up something a bit muddled about Windows networked drives. ## It helps to think of share points as being like drive letters. Example: ## k: is not a directory ## k:\ is a directory ## k:\media is a directory ## k:\media\ is not a directory ## The same rules apply if you substitute "k:" with "\\conky\foo": ## \\conky\foo is not a directory ## \\conky\foo\ is a directory ## \\conky\foo\media is a directory ## \\conky\foo\media\ is not a directory ## Edit this entry / Log info / Last changed on Sun Jan 31 08:44:48 1999 by GvR ## -------------------------------------------------------------------------------- ## 8.7. PyRun_SimpleFile() crashes on Windows but not on Unix ## I've seen a number of reports of PyRun_SimpleFile() failing in a Windows port of an application embedding Python that worked fine on Unix. PyRun_SimpleString() works fine on both platforms. ## I think this happens because the application was compiled with a different set of compiler flags than Python15.DLL. It seems that some compiler flags affect the standard I/O library in such a way that using different flags makes calls fail. You need to set it for the non-debug multi-threaded DLL (/MD on the command line, or can be set via MSVC under Project Settings->C++/Code Generation then the "Use rum-time library" dropdown.) ## Also note that you can not mix-and-match Debug and Release versions. If you wish to use the Debug Multithreaded DLL, then your module _must_ have an "_d" appended to the base name. ## Edit this entry / Log info / Last changed on Wed Nov 17 17:37:07 1999 by Mark Hammond ## -------------------------------------------------------------------------------- ## 8.8. Import of _tkinter fails on Windows 95/98 ## Sometimes, the import of _tkinter fails on Windows 95 or 98, complaining with a message like the following: ## ImportError: DLL load failed: One of the library files needed ## to run this application cannot be found. ## It could be that you haven't installed Tcl/Tk, but if you did install Tcl/Tk, and the Wish application works correctly, the problem may be that its installer didn't manage to edit the autoexec.bat file correctly. It tries to add a statement that changes the PATH environment variable to include the Tcl/Tk 'bin' subdirectory, but sometimes this edit doesn't quite work. Opening it with notepad usually reveals what the problem is. ## (One additional hint, noted by David Szafranski: you can't use long filenames here; e.g. use C:\PROGRA~1\Tcl\bin instead of C:\Program Files\Tcl\bin.) ## Edit this entry / Log info / Last changed on Wed Dec 2 22:32:41 1998 by GvR ## -------------------------------------------------------------------------------- ## 8.9. Can't extract the downloaded documentation on Windows ## Sometimes, when you download the documentation package to a Windows machine using a web browser, the file extension of the saved file ends up being .EXE. This is a mistake; the extension should be .TGZ. ## Simply rename the downloaded file to have the .TGZ extension, and WinZip will be able to handle it. (If your copy of WinZip doesn't, get a newer one from http://www.winzip.com.) ## Edit this entry / Log info / Last changed on Sat Nov 21 13:41:35 1998 by GvR ## -------------------------------------------------------------------------------- ## 8.10. Can't get Py_RunSimpleFile() to work. ## This is very sensitive to the compiler vendor, version and (perhaps) even options. If the FILE* structure in your embedding program isn't the same as is assumed by the Python interpreter it won't work. ## The Python 1.5.* DLLs (python15.dll) are all compiled with MS VC++ 5.0 and with multithreading-DLL options (/MD, I think). ## If you can't change compilers or flags, try using Py_RunSimpleString(). A trick to get it to run an arbitrary file is to construct a call to execfile() with the name of your file as argument. ## Edit this entry / Log info / Last changed on Wed Jan 13 10:58:14 1999 by GvR ## -------------------------------------------------------------------------------- ## 8.11. Where is Freeze for Windows? ## ("Freeze" is a program that allows you to ship a Python program as a single stand-alone executable file. It is not a compiler, your programs don't run any faster, but they are more easily distributable (to platforms with the same OS and CPU). Read the README file of the freeze program for more disclaimers.) ## You can use freeze on Windows, but you must download the source tree (see http://www.python.org/download/download_source.html). This is recommended for Python 1.5.2 (and betas thereof) only; older versions don't quite work. ## You need the Microsoft VC++ 5.0 compiler (maybe it works with 6.0 too). You probably need to build Python -- the project files are all in the PCbuild directory. ## The freeze program is in the Tools\freeze subdirectory of the source tree. ## Edit this entry / Log info / Last changed on Wed Feb 17 18:47:24 1999 by GvR ## -------------------------------------------------------------------------------- ## 8.12. Is a *.pyd file the same as a DLL? ## Yes, .pyd files are dll's. But there are a few differences. If you have a DLL named foo.pyd, then it must have a function initfoo(). You can then write Python "import foo", and Python will search for foo.pyd (as well as foo.py, foo.pyc) and if it finds it, will attempt to call initfoo() to initialize it. You do not link your .exe with foo.lib, as that would cause Windows to require the DLL to be present. ## Note that the search path for foo.pyd is PYTHONPATH, not the same as the path that Windows uses to search for foo.dll. Also, foo.pyd need not be present to run your program, whereas if you linked your program with a dll, the dll is required. Of course, foo.pyd is required if you want to say "import foo". In a dll, linkage is declared in the source code with __declspec(dllexport). In a .pyd, linkage is defined in a list of available functions. ## Edit this entry / Log info / Last changed on Tue Nov 23 02:40:08 1999 by Jameson Quinn ## -------------------------------------------------------------------------------- ## 8.13. Missing cw3215mt.dll (or missing cw3215.dll) ## Sometimes, when using Tkinter on Windows, you get an error that cw3215mt.dll or cw3215.dll is missing. ## Cause: you have an old Tcl/Tk DLL built with cygwin in your path (probably C:\Windows). You must use the Tcl/Tk DLLs from the standard Tcl/Tk installation (Python 1.5.2 comes with one). ## Edit this entry / Log info / Last changed on Fri Jun 11 00:54:13 1999 by GvR ## -------------------------------------------------------------------------------- ## 8.14. How to make python scripts executable: ## [Blake Coverett] ## Win2K: ## The standard installer already associates the .py extension with a file type (Python.File) and gives that file type an open command that runs the interpreter (D:\Program Files\Python\python.exe "%1" %*). This is enough to make scripts executable from the command prompt as 'foo.py'. If you'd rather be able to execute the script by simple typing 'foo' with no extension you need to add .py to the PATHEXT environment variable. ## WinNT: ## The steps taken by the installed as described above allow you do run a script with 'foo.py', but a long time bug in the NT command processor prevents you from redirecting the input or output of any script executed in this way. This is often important. ## An appropriate incantation for making a Python script executable under WinNT is to give the file an extension of .cmd and add the following as the first line: ## @setlocal enableextensions & python -x %~f0 %* & goto :EOF ## Win9x: ## [Due to Bruce Eckel] ## @echo off ## rem = """ ## rem run python on this bat file. Needs the full path where ## rem you keep your python files. The -x causes python to skip ## rem the first line of the file: ## python -x c:\aaa\Python\\"%0".bat %1 %2 %3 %4 %5 %6 %7 %8 %9 ## goto endofpython ## rem """ ## # The python program goes here: ## print "hello, Python" ## # For the end of the batch file: ## rem = """ ## :endofpython ## rem """ ## Edit this entry / Log info / Last changed on Tue Nov 30 10:25:17 1999 by GvR ## -------------------------------------------------------------------------------- ## 8.15. Warning about CTL3D32 version from installer ## The Python installer issues a warning like this: ## This version uses CTL3D32.DLL whitch is not the correct version. ## This version is used for windows NT applications only. ## [Tim Peters] This is a Microsoft DLL, and a notorious source of problems. The msg means what it says: you have the wrong version of this DLL for your operating system. The Python installation did not cause this -- something else you installed previous to this overwrote the DLL that came with your OS (probably older shareware of some sort, but there's no way to tell now). If you search for "CTL3D32" using any search engine (AltaVista, for example), you'll find hundreds and hundreds of web pages complaining about the same problem with all sorts of installation programs. They'll point you to ways to get the correct version reinstalled on your system (since Python doesn't cause this, we can't fix it). ## David A Burton has written a little program to fix this. Go to http://www.burtonsys.com/download.html and click on "ctl3dfix.zip" ## Edit this entry / Log info / Last changed on Thu Oct 26 15:42:00 2000 by GvR ## -------------------------------------------------------------------------------- ## 8.16. How can I embed Python into a Windows application? ## Edward K. Ream writes ## When '##' appears in a file name below, it is an abbreviated version number. For example, for Python 2.1.1, ## will be replaced by 21. ## Embedding the Python interpreter in a Windows app can be summarized as follows: ## 1. Do _not_ build Python into your .exe file directly. On Windows, Python must be a DLL to handle importing modules that are themselves DLL's. (This is the first key undocumented fact.) Instead, link to python##.dll; it is typically installed in c:\Windows\System. ## You can link to Python statically or dynamically. Linking statically means linking against python##.lib The drawback is that your app won't run if python##.dll does not exist on your system. ## General note: python##.lib is the so-called "import lib" corresponding to python.dll. It merely defines symbols for the linker. ## Borland note: convert python##.lib to OMF format using Coff2Omf.exe first. ## Linking dynamically greatly simplifies link options; everything happens at run time. Your code must load python##.dll using the Windows LoadLibraryEx() routine. The code must also use access routines and data in python##.dll (that is, Python's C API's) using pointers obtained by the Windows GetProcAddress() routine. Macros can make using these pointers transparent to any C code that calls routines in Python's C API. ## 2. If you use SWIG, it is easy to create a Python "extension module" that will make the app's data and methods available to Python. SWIG will handle just about all the grungy details for you. The result is C code that you link _into your .exe file_ (!) You do _not_ have to create a DLL file, and this also simplifies linking. ## 3. SWIG will create an init function (a C function) whose name depends on the name of the extension module. For example, if the name of the module is leo, the init function will be called initleo(). If you use SWIG shadow classes, as you should, the init function will be called initleoc(). This initializes a mostly hidden helper class used by the shadow class. ## The reason you can link the C code in step 2 into your .exe file is that calling the initialization function is equivalent to importing the module into Python! (This is the second key undocumented fact.) ## 4. In short, you can use the following code to initialize the Python interpreter with your extension module. ## #include "python.h" ## ... ## Py_Initialize(); // Initialize Python. ## initmyAppc(); // Initialize (import) the helper class. ## PyRun_SimpleString("import myApp") ; // Import the shadow class. ## 5. There are two problems with Python's C API which will become apparent if you use a compiler other than MSVC, the compiler used to build python##.dll. ## Problem 1: The so-called "Very High Level" functions that take FILE * arguments will not work in a multi-compiler environment; each compiler's notion of a struct FILE will be different. From an implementation standpoint these are very _low_ level functions. ## Problem 2: SWIG generates the following code when generating wrappers to void functions: ## Py_INCREF(Py_None); ## _resultobj = Py_None; ## return _resultobj; ## Alas, Py_None is a macro that expands to a reference to a complex data structure called _Py_NoneStruct inside python##.dll. Again, this code will fail in a mult-compiler environment. Replace such code by: ## return Py_BuildValue(""); ## It may be possible to use SWIG's %typemap command to make the change automatically, though I have not been able to get this to work (I'm a complete SWIG newbie). ## 6. Using a Python shell script to put up a Python interpreter window from inside your Windows app is not a good idea; the resulting window will be independent of your app's windowing system. Rather, you (or the wxPythonWindow class) should create a "native" interpreter window. It is easy to connect that window to the Python interpreter. You can redirect Python's i/o to _any_ object that supports read and write, so all you need is a Python object (defined in your extension module) that contains read() and write() methods. ## Edit this entry / Log info / Last changed on Thu Jan 31 16:29:34 2002 by Victor Kryukov ## -------------------------------------------------------------------------------- ## 8.17. Setting up IIS 5 to use Python for CGI ## In order to set up Internet Information Services 5 to use Python for CGI processing, please see the following links: ## http://www.e-coli.net/pyiis_server.html (for Win2k Server) http://www.e-coli.net/pyiis.html (for Win2k pro) ## Edit this entry / Log info / Last changed on Fri Mar 22 22:05:51 2002 by douglas savitsky ## -------------------------------------------------------------------------------- ## 8.18. How do I run a Python program under Windows? ## This is not necessarily quite the straightforward question it appears to be. If you are already familiar with running programs from the Windows command line then everything will seem really easy and obvious. If your computer experience is limited then you might need a little more guidance. Also there are differences between Windows 95, 98, NT, ME, 2000 and XP which can add to the confusion. You might think of this as "why I pay software support charges" if you have a helpful and friendly administrator to help you set things up without having to understand all this yourself. If so, then great! Show them this page and it should be a done deal. ## Unless you use some sort of integrated development environment (such as PythonWin or IDLE, to name only two in a growing family) then you will end up typing Windows commands into what is variously referred to as a "DOS window" or "Command prompt window". Usually you can create such a window from your Start menu (under Windows 2000 I use "Start | Programs | Accessories | Command Prompt"). You should be able to recognize when you have started such a window because you will see a Windows "command prompt", which usually looks like this: ## C:\> ## The letter may be different, and there might be other things after it, so you might just as easily see something like: ## D:\Steve\Projects\Python> ## depending on how your computer has been set up and what else you have recently done with it. Once you have started such a window, you are well on the way to running Python programs. ## You need to realize that your Python scripts have to be processed by another program, usually called the "Python interpreter". The interpreter reads your script, "compiles" it into "Python bytecodes" (which are instructions for an imaginary computer known as the "Python Virtual Machine") and then executes the bytecodes to run your program. So, how do you arrange for the interpreter to handle your Python? ## First, you need to make sure that your command window recognises the word "python" as an instruction to start the interpreter. If you have opened a command window, you should try entering the command: ## python ## and hitting return. If you then see something like: ## Python 2.2 (#28, Dec 21 2001, 12:21:22) [MSC 32 bit (Intel)] on win32 ## Type "help", "copyright", "credits" or "license" for more information. ## >>> ## then this part of the job has been correctly managed during Python's installation process, and you have started the interpreter in "interactive mode". That means you can enter Python statements or expressions interactively and have them executed or evaluated while you wait. This is one of Python's strongest features, but it takes a little getting used to. Check it by entering a few expressions of your choice and seeing the results... ## >>> print "Hello" ## Hello ## >>> "Hello" * 3 ## HelloHelloHello ## When you want to end your interactive Python session, enter a terminator (hold the Ctrl key down while you enter a Z, then hit the "Enter" key) to get back to your Windows command prompt. You may also find that you have a Start-menu entry such as "Start | Programs | Python 2.2 | Python (command line)" that results in you seeing the ">>>" prompt in a new window. If so, the window will disappear after you enter the terminator -- Windows runs a single "python" command in the window, which terminates when you terminate the interpreter. ## If the "python" command, instead of displaying the interpreter prompt ">>>", gives you a message like ## 'python' is not recognized as an internal or external command, ## operable program or batch file. ## or ## Bad command or filename ## then you need to make sure that your computer knows where to find the Python interpreter. To do this you will have to modify a setting called the PATH, which is a just list of directories where Windows will look for programs. Rather than just enter the right command every time you create a command window, you should arrange for Python's installation directory to be added to the PATH of every command window as it starts. If you installed Python fairly recently then the command ## dir C:\py* ## will probably tell you where it is installed. Alternatively, perhaps you made a note. Otherwise you will be reduced to a search of your whole disk ... break out the Windows explorer and use "Tools | Find" or hit the "Search" button and look for "python.exe". Suppose you discover that Python is installed in the C:\Python22 directory (the default at the time of writing) then you should make sure that entering the command ## c:\Python22\python ## starts up the interpreter as above (and don't forget you'll need a "CTRL-Z" and an "Enter" to get out of it). Once you have verified the directory, you need to add it to the start-up routines your computer goes through. For older versions of Windows the easiest way to do this is to edit the C:\AUTOEXEC.BAT file. You would want to add a line like the following to AUTOEXEC.BAT: ## PATH C:\Python22;%PATH% ## For Windows NT, 2000 and (I assume) XP, you will need to add a string such as ## ;C:\Python22 ## to the current setting for the PATH environment variable, which you will find in the properties window of "My Computer" under the "Advanced" tab. Note that if you have sufficient privilege you might get a choice of installing the settings either for the Current User or for System. The latter is preferred if you want everybody to be able to run Python on the machine. ## If you aren't confident doing any of these manipulations yourself, ask for help! At this stage you may or may not want to reboot your system to make absolutely sure the new setting has "taken" (don't you love the way Windows gives you these freqeuent coffee breaks). You probably won't need to for Windows NT, XP or 2000. You can also avoid it in earlier versions by editing the file C:\WINDOWS\COMMAND\CMDINIT.BAT instead of AUTOEXEC.BAT. ## You should now be able to start a new command window, enter ## python ## at the "C:>" (or whatever) prompt, and see the ">>>" prompt that indicates the Python interpreter is reading interactive commands. ## Let's suppose you have a program called "pytest.py" in directory "C:\Steve\Projects\Python". A session to run that program might look like this: ## C:\> cd \Steve\Projects\Python ## C:\Steve\Projects\Python> python pytest.py ## Because you added a file name to the command to start the interpreter, when it starts up it reads the Python script in the named file, compiles it, executes it, and terminates (so you see another "C:\>" prompt). You might also have entered ## C:\> python \Steve\Projects\Python\pytest.py ## if you hadn't wanted to change your current directory. ## Under NT, 2000 and XP you may well find that the installation process has also arranged that the command ## pytest.py ## (or, if the file isn't in the current directory) ## C:\Steve\Projects\Python\pytest.py ## will automatically recognize the ".py" extension and run the Python interpreter on the named file. Using this feature is fine, but some versions of Windows have bugs which mean that this form isn't exactly equivalent to using the interpreter explicitly, so be careful. Easier to remember, for now, that ## python C:\Steve\Projects\Python\pytest.py ## works pretty close to the same, and redirection will work (more) reliably. ## The important things to remember are: ## 1. Start Python from the Start Menu, or make sure the PATH is set correctly so Windows can find the Python interpreter. ## python ## should give you a '>>>" prompt from the Python interpreter. Don't forget the CTRL-Z and ENTER to terminate the interpreter (and, if you started the window from the Start Menu, make the window disappear). ## 2. Once this works, you run programs with commands: ## python {program-file} ## 3. When you know the commands to use you can build Windows shortcuts to run the Python interpreter on any of your scripts, naming particular working directories, and adding them to your menus, but that's another lessFAQ. Take a look at ## python --help ## if your needs are complex. ## 4. Interactive mode (where you see the ">>>" prompt) is best used not for running programs, which are better executed as in steps 2 and 3, but for checking that individual statements and expressions do what you think they will, and for developing code by experiment. ## Edit this entry / Log info / Last changed on Tue Aug 20 16:19:53 2002 by GvR ## -------------------------------------------------------------------------------- ## Python home / Python FAQ Wizard 1.0.3 / Feedback to GvR