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python中'@'符号用作函数修饰符

2008-08-24 14:10 267 查看

'@'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 "<stdin>", 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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