【转】 python中的 @ 修饰符
原文地址：http://blog.csdn.net/lainegates/article/details/8166764今天看到python中的一个修饰符'@'，不了解它的使用，查看了下官方文档，有了一点了解。
原文 PEP-318 网址：http://www.python.org/dev/peps/pep-0318/
不得不佩服老外，治学很严谨，在python网站相关网页上把为什么使用decorator（主要为了简便一些代码），以及使用什么字符，甚至语法怎么设计写了个详详细细，好长的一篇啊。
这是查看的其中一篇，我翻译关键部分的一些内容，又摘取一些有用的，有空再翻译。
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[*]@dec2
[*]@dec1
[*]def func(arg1, arg2, ...):
[*] pass
This is equivalent to（等价于）:
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[*]def func(arg1, arg2, ...):
[*] pass
[*]func = dec2(dec1(func))
使用示例：
Much of the discussion on comp.lang.python and the python-dev mailing list focuses on the use of decorators as a cleaner way to use the staticmethod() and classmethod() builtins. This capability is much more powerful than that. This section presents some examples of use.
在comp.lang.python 和 python-dev的大部分讨论集中在更简捷地使用内置修饰符staticmethod() 和 classmethod() 上。但修饰符的功能远比这强大。下面会对它的使用进行一些讲解：
1.Define a function to be executed at exit. Note that the function isn't actually "wrapped" in the usual sense.
1.定义一个执行即退出的函数。注意，这个函数并不像通常情况那样，被真正包裹。
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[*]def onexit(f):
[*] import atexit
[*] atexit.register(f)
[*] return f
[*]
[*]@onexit
[*]def func():
[*] ...
Note that this example is probably not suitable for real usage, but is for example purposes only.
注意，这个示例可能并不能准确表达在实际中的使用，它只是做一个示例。
2. Define a class with a singleton instance. Note that once the class disappears enterprising programmers would have to be more creative to create more instances. (From Shane Hathaway onpython-dev.)
2.定义一个只能产生一个实例的类（有实例后，这个类不能再产生新的实例）。注意，一旦这个类失效了(估计意思是保存在下文的singleton中字典中的相应键失效)，就会促使程序员让这个类产生更多的实例。（来自于python-dev的Shane Hathaway）
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[*]def singleton(cls):
[*] instances = {}
[*] def getinstance():
[*] if cls not in instances:
[*] instances[cls] = cls()
[*] return instances[cls]
[*] return getinstance
[*]
[*]@singleton
[*]class MyClass:
[*] ...
余下基本可以参照着读懂了，以后再翻译。
3.Add attributes to a function. (Based on an example posted by Anders Munch on python-dev.)
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[*]def attrs(**kwds):
[*] def decorate(f):
[*] for k in kwds:
[*] setattr(f, k, kwds)
[*] return f
[*] return decorate
[*]
[*]@attrs(versionadded="2.2",
[*] author="Guido van Rossum")
[*]def mymethod(f):
[*] ...
4.Enforce function argument and return types. Note that this copies the func_name attribute from the old to the new function. func_name was made writable in Python 2.4a3:
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[*]def accepts(*types):
[*] def check_accepts(f):
[*] assert len(types) == f.func_code.co_argcount
[*] def new_f(*args, **kwds):
[*] for (a, t) in zip(args, types):
[*] assert isinstance(a, t), \
[*] "arg %r does not match %s" % (a,t)
[*] return f(*args, **kwds)
[*] new_f.func_name = f.func_name
[*] return new_f
[*] return check_accepts
[*]
[*]def returns(rtype):
[*] def check_returns(f):
[*] def new_f(*args, **kwds):
[*] result = f(*args, **kwds)
[*] assert isinstance(result, rtype), \
[*] "return value %r does not match %s" % (result,rtype)
[*] return result
[*] new_f.func_name = f.func_name
[*] return new_f
[*] return check_returns
[*]
[*]@accepts(int, (int,float))
[*]@returns((int,float))
[*]def func(arg1, arg2):
[*] return arg1 * arg2
5.Declare that a class implements a particular (set of) interface(s). This is from a posting by Bob Ippolito on python-dev based on experience with PyProtocols .
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[*]def provides(*interfaces):
[*] """
[*] An actual, working, implementation of provides for
[*] the current implementation of PyProtocols. Not
[*] particularly important for the PEP text.
[*] """
[*] def provides(typ):
[*] declareImplementation(typ, instancesProvide=interfaces)
[*] return typ
[*] return provides
[*]
[*]class IBar(Interface):
[*] """Declare something about IBar here"""
[*]
[*]@provides(IBar)
[*]class Foo(object):
[*] """Implement something here..."""
Of course, all these examples are possible today, though without syntactic support.
df PSdf
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