Subsections


9. Brief Tour of the Standard Library 标准库概览


9.1 Operating System Interface 操作系统接口

The os module provides dozens of functions for interacting with the operating system:

os 模块提供了不少与操作系统相关联的函数。

>>> import os
>>> os.system('time 0:02')
0
>>> os.getcwd()      # Return the current working directory
'C:\\Python24'
>>> os.chdir('/server/accesslogs')

Be sure to use the "import os" style instead of "from os import *". This will keep os.open() from shadowing the builtin open() function which operates much differently.

应该用 "import os" 风格而非 "from os import *"。这样可以保证随操作系统不同而有所变化的 os.open() 不会覆盖内置函数 open()

The builtin dir() and help() functions are useful as interactive aids for working with large modules like os:

在使用一些像 os 这样的大型模块时内置的 dir()help() 函数非常有用。

>>> import os
>>> dir(os)
<returns a list of all module functions>
>>> help(os)
<returns an extensive manual page created from the module's docstrings>

For daily file and directory management tasks, the shutil module provides a higher level interface that is easier to use:

针对日常的文件和目录管理任务,shutil 模块提供了一个易于使用的高级接口。

>>> import shutil
>>> shutil.copyfile('data.db', 'archive.db')
>>> shutil.move('/build/executables', 'installdir')


9.2 File Wildcards 文件通配符

The glob module provides a function for making file lists from directory wildcard searches:

glob 模块提供了一个函数用于从目录通配符搜索中生成文件列表。

>>> import glob
>>> glob.glob('*.py')
['primes.py', 'random.py', 'quote.py']


9.3 Command Line Arguments 命令行参数

Common utility scripts often need to process command line arguments. These arguments are stored in the sys module's argv attribute as a list. For instance the following output results from running "python demo.py one two three" at the command line:

通用工具脚本经常调用命令行参数。这些命令行参数以链表形式存储于 sys 模块的 argv 变量。例如在命令行中执行 "python demo.py one two three" 后可以得到以下输出结果:

>>> import sys
>>> print sys.argv
['demo.py', 'one', 'two', 'three']

The getopt module processes sys.argv using the conventions of the Unix getopt() function. More powerful and flexible command line processing is provided by the optparse module.

getopt 模块使用 Unix getopt() 函处理 sys.argv。更多的复杂命令行处理由 optparse 模块提供。


9.4 Error Output Redirection and Program Termination 错误输出重定向和程序终止

The sys module also has attributes for stdin, stdout, and stderr. The latter is useful for emitting warnings and error messages to make them visible even when stdout has been redirected:

sys 还有 stdinstdoutstderr 属性,即使在 stdout 被重定向时,后者也可以用于显示警告和错误信息。

>>> sys.stderr.write('Warning, log file not found starting a new one\n')
Warning, log file not found starting a new one

The most direct way to terminate a script is to use "sys.exit()".

大多脚本的定向终止都使用 "sys.exit()"。


9.5 String Pattern Matching 字符串正则匹配

The re module provides regular expression tools for advanced string processing. For complex matching and manipulation, regular expressions offer succinct, optimized solutions:

re 模块为高级字符串处理提供了正则表达式工具。对于复杂的匹配和处理,正则表达式提供了简洁、优化的解决方案。

>>> import re
>>> re.findall(r'\bf[a-z]*', 'which foot or hand fell fastest')
['foot', 'fell', 'fastest']
>>> re.sub(r'(\b[a-z]+) \1', r'\1', 'cat in the the hat')
'cat in the hat'

When only simple capabilities are needed, string methods are preferred because they are easier to read and debug:

如果只需要简单的功能,应该首先考虑字符串方法,因为它们非常简单,易于阅读和调试。

>>> 'tea for too'.replace('too', 'two')
'tea for two'


9.6 Mathematics 数学

The math module gives access to the underlying C library functions for floating point math:

math 模块为浮点运算提供了对底层C函数库的访问。

>>> import math
>>> math.cos(math.pi / 4.0)
0.70710678118654757
>>> math.log(1024, 2)
10.0

The random module provides tools for making random selections:

random 提供了生成随机数的工具。

>>> import random
>>> random.choice(['apple', 'pear', 'banana'])
'apple'
>>> random.sample(xrange(100), 10)   # sampling without replacement
[30, 83, 16, 4, 8, 81, 41, 50, 18, 33]
>>> random.random()    # random float
0.17970987693706186
>>> random.randrange(6)    # random integer chosen from range(6)
4


9.7 Internet Access 互联网访问

There are a number of modules for accessing the internet and processing internet protocols. Two of the simplest are urllib2 for retrieving data from urls and smtplib for sending mail:

有几个模块用于访问互联网以及处理网络通信协议。其中最简单的两个是用于处理从 urls 接收的数据的 urllib2 以及用于发送电子邮件的 smtplib

>>> import urllib2
>>> for line in urllib2.urlopen('http://tycho.usno.navy.mil/cgi-bin/timer.pl'):
...     if 'EST' in line or 'EDT' in line:  # look for Eastern Time
...         print line
    
<BR>Nov. 25, 09:43:32 PM EST

>>> import smtplib
>>> server = smtplib.SMTP('localhost')
>>> server.sendmail('soothsayer@example.org', 'jcaesar@example.org',
"""To: jcaesar@example.org
From: soothsayer@example.org

Beware the Ides of March.
""")
>>> server.quit()


9.8 Dates and Times 日期和时间

The datetime module supplies classes for manipulating dates and times in both simple and complex ways. While date and time arithmetic is supported, the focus of the implementation is on efficient member extraction for output formatting and manipulation. The module also supports objects that are timezone aware.

datetime 模块为日期和时间处理同时提供了简单和复杂的方法。支持日期和时间算法的同时,实现的重点放在更有效的处理和格式化输出。该模块还支持时区处理。

# dates are easily constructed and formatted
>>> from datetime import date
>>> now = date.today()
>>> now
datetime.date(2003, 12, 2)
>>> now.strftime("%m-%d-%y. %d %b %Y is a %A on the %d day of %B.")
'12-02-03. 02 Dec 2003 is a Tuesday on the 02 day of December.'

# dates support calendar arithmetic
>>> birthday = date(1964, 7, 31)
>>> age = now - birthday
>>> age.days
14368


9.9 Data Compression 数据压缩

Common data archiving and compression formats are directly supported by modules including: zlib, gzip, bz2, zipfile, and tarfile.

以下模块直接支持通用的数据打包和压缩格式:

zlibgzipbz2zipfile, 以及 tarfile

>>> import zlib
>>> s = 'witch which has which witches wrist watch'
>>> len(s)
41
>>> t = zlib.compress(s)
>>> len(t)
37
>>> zlib.decompress(t)
'witch which has which witches wrist watch'
>>> zlib.crc32(s)
226805979


9.10 Performance Measurement 性能度量

Some Python users develop a deep interest in knowing the relative performance of different approaches to the same problem. Python provides a measurement tool that answers those questions immediately.

有些用户对了解解决同一问题的不同方法之间的性能差异很感兴趣。Python 提供了一个度量工具,为这些问题提供了直接答案。

For example, it may be tempting to use the tuple packing and unpacking feature instead of the traditional approach to swapping arguments. The timeit module quickly demonstrates a modest performance advantage:

例如,使用元组封装和拆封来交换元素看起来要比使用传统的方法要诱人的多。timeit 证明了传统的方法更快一些。

>>> from timeit import Timer
>>> Timer('t=a; a=b; b=t', 'a=1; b=2').timeit()
0.57535828626024577
>>> Timer('a,b = b,a', 'a=1; b=2').timeit()
0.54962537085770791

In contrast to timeit's fine level of granularity, the profile and pstats modules provide tools for identifying time critical sections in larger blocks of code.

相对于 timeit 的细粒度,profilepstats 模块提供了针对更大代码块的时间度量工具。


9.11 Quality Control 质量控制

One approach for developing high quality software is to write tests for each function as it is developed and to run those tests frequently during the development process.

开发高质量软件的方法之一是为每一个函数开发测试代码,并且在开发过程中经常进行测试。

The doctest module provides a tool for scanning a module and validating tests embedded in a program's docstrings. Test construction is as simple as cutting-and-pasting a typical call along with its results into the docstring. This improves the documentation by providing the user with an example and it allows the doctest module to make sure the code remains true to the documentation:

doctest 模块提供了一个工具,扫描模块并根据程序中内嵌的文档字符串执行测试。测试构造如同简单的将它的输出结果剪切并粘贴到文档字符串中。通过用户提供的例子,它发展了文档,允许 doctest 模块确认代码的结果是否与文档一致。

def average(values):
    """Computes the arithmetic mean of a list of numbers.

    >>> print average([20, 30, 70])
    40.0
    """
    return sum(values, 0.0) / len(values)

import doctest
doctest.testmod()   # automatically validate the embedded tests

The unittest module is not as effortless as the doctest module, but it allows a more comprehensive set of tests to be maintained in a separate file:

unittest 模块不像 doctest 模块那么容易使用,不过它可以在一个独立的文件里提供一个更全面的测试集。

import unittest

class TestStatisticalFunctions(unittest.TestCase):

    def test_average(self):
        self.assertEqual(average([20, 30, 70]), 40.0)
        self.assertEqual(round(average([1, 5, 7]), 1), 4.3)
        self.assertRaises(ZeroDivisionError, average, [])
        self.assertRaises(TypeError, average, 20, 30, 70)

unittest.main() # Calling from the command line invokes all tests


9.12 Batteries Included

Python has a ``batteries included'' philosophy. This is best seen through the sophisticated and robust capabilities of its larger packages. For example:

Python 体现了“batteries included”哲学。Python 可以通过更大的包的来得到应付各种复杂情况的强大能力,从这一点我们可以看出该思想的应用。例如:

See About this document... for information on suggesting changes.