python中的 @ 修饰符

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。

    @dec2  
    @dec1  
    def func(arg1, arg2, ...):  
        pass  

This is equivalent to（等价于）:

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。

    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）

    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.)

    def attrs(**kwds):  
        def decorate(f):  
            for k in kwds:  
                setattr(f, k, kwds[k])  
            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:

    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 [27].

    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.