python中'@'符号用作函数修饰符

    '@'符号用作函数修饰符是python2.4新增加的功能，修饰符必须出现在函数定义前一行，不允许和函数定义在同一行。也就是说 ＠A def f(): 是非法的。 只可以在模块或类定义层内对函数进行修饰，不允许修修饰一个类。 一个修饰符就是一个函数，它将被修饰的函数做为参数，并返回修饰后的同名函数或其它可调用的东西。
    请看以下实例： 

    >>> def spamrun(fn):
    ...     def sayspam(*args):
    ...         print "spam, spam, spam"
    ...     return sayspam
    ...
    >>> @spamrun
    ... def useful(a, b):
    ...     print a**2 + b**2
    ...
    >>> useful(3,4)
    spam, spam, spam

请参考python主页中的文档：http://www.python.org/dev/peps/pep-0318/，相信会对修饰符有一个更深入的认识。

更多例子：
Listing 1. Bad decorator that does not even return function

>>> def spamdef(fn): ... print "spam, spam, spam" ... >>> @spamdef ... def useful(a, b): ... print a**2 + b**2 ... spam, spam, spam >>> useful(3, 4) Traceback (most recent call last): File "", line 1, in ? TypeError: 'NoneType' object is not callable

A decorator might return a function, but one that is not meaningfully associated with the undecorated function:

Listing 2. Decorator whose function ignores passed-in function

>>> def spamrun(fn): ... def sayspam(*args): ... print "spam, spam, spam" ... return sayspam ... >>> @spamrun ... def useful(a, b): ... print a**2 + b**2 ... >>> useful(3,4) spam, spam, spam

Finally, a better behaved decorator will in some way enhance or modify the action of the undecorated function:

Listing 3. Decorator that modifies behavior of undecorated func

        
>>> def addspam(fn):
...     def new(*args):
...         print "spam, spam, spam"
...         return fn(*args)
...     return new
...
>>> @addspam
... def useful(a, b):
...     print a**2 + b**2
...
>>> useful(3,4)
spam, spam, spam

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