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== 1.4 Getting a Value from a Dictionary ==
从字典中获取一个值

Credit: Andy McKay

=== 1.4.1 Problem ===
问题

You need to obtain a value from a dictionary, without having to handle an exception if the key you seek is not in the dictionary.

你需要从字典中获得一个值，不要处理在字典里找不到你所需要的键值的异常。

=== 1.4.2 Solution ===
That's what the get method of dictionaries is for. Say you have a dictionary:

那就是字典的get方法。 如果你有一个字典

{{{
#!python
d = {'key':'value'}
}}}
You can write a test to pull out the value of 'key' from d in an exception-safe way:

在一个异常安全的方法中，你能够写一个从d中提取‘key’的值的测试

{{{
#!python
if d.has_key('key'):              # or, in Python 2.2 or later: if 'key' in d:
    print d['key']
else:
    print 'not found'
}}}
However, there is a much simpler syntax:

然而有一个更简单的方法

{{{
#!python
print d.get('key', 'not found')
}}}
=== 1.4.3 Discussion ===
讨论

Want to get a value from a dictionary but first make sure that the value exists in the dictionary? Use the simple and useful get method.

想从一个字典获得一个值，但是首先要确信这个值是否在这个字典里？使用简单有效的get方法。

If you try to get a value with a syntax such as d[x], and the value of x is not a key in dictionary d, your attempt raises a KeyError exception. This is often okay. If you expected the value of x to be a key in d, an exception is just the right way to inform you that you're wrong (i.e., that you need to debug your program).

如果你试着用象d[x]那样的语法来获得一个值，并且x的值不是字典d的键值， 你的尝试将抛出一个KeyError异常。 这个是经常有用的。如果你期望x的值是d中的一个键值，一个异常是通知你犯错了的正确途径。（那就是说，你需要调试你的程序了）

However, you often need to be more tentative about it: as far as you know, the value of x may or may not be a key in d. In this case, don't start messing with the has_key method or with try/except statements. Instead, use the get method. If you call d.get(x), no exception is thrown: you get d[x] if x is a key in d, and if it's not, you get None (which you can check for or propagate). If None is not what you want to get when x is not a key of d, call d.get(x, somethingelse) instead. In this case, if x is not a key, you will get the value of somethingelse.

然而，关于它，你经常需要更多的假设：直到你知道x的值或者是或者不是d中的一个键值。在这种情况下，忘掉has_key方法或者try/except语句。 取而代之的，使用get方法。如果你调用d.get(x)，没有异常被抛出。如果x是d的一个键值，你得到d[x]。如果不是，你得到None（你能检查或者传播它）。 当x不是d的键值的时候，如果None不是你想要的，调用d.get(x, somethingelse)来替代。在这种情况下，如果x不是一个键值。你将得到somethingelse的值

get is a simple, useful mechanism that is well explained in the Python documentation, but a surprising number of people don't know about it. This idiom is also quite common in Zope, for example, when pulling variables out of the REQUEST dictionary.

get是简单，有效的机制，python的文档很好的解释了它。但是不知道它的人的数量令人惊讶。这个惯用方法在zope中也是相当普遍的，比如抽取REQUEST字典的一些值的时候。

=== 1.4.4 See Also ===
参考

The Library Reference section on mapping types.