python 理解functools.wraps

先复习下装饰器

# coding=utf-8def logged(func):    def with_logging(*args, **kwargs):        """        哈哈哈，这里是with_logging        :param args:        :param kwargs:        :return:        """        print func.__name__, " was called..."        return func(*args, **kwargs)    return with_logging@loggeddef f(x):    """    哈哈，这是f    :param x:    :return:    """    return x + x * xprint f.__name__print f.__doc__

打印结果是：

(venv) ➜  myApp git:(v1.6_dev) ✗ python 1.pywith_logging        哈哈哈，这里是with_logging        :param args:        :param kwargs:        :return:

理论上这是不对的，因为函数f的信息已经获取不到了。。都被with_logging取代了。

解决方案：

# coding=utf-8import functoolsdef logged(func):    @functools.wraps(func)    def with_logging(*args, **kwargs):        """        哈哈哈，这里是with_logging        :param args:        :param kwargs:        :return:        """        print func.__name__, " was called..."        return func(*args, **kwargs)    return with_logging@loggeddef f(x):    """    哈哈，这是f    :param x:    :return:    """    return x + x * xprint f.__name__print f.__doc__

打印结果是：

(venv) ➜  myApp git:(v1.6_dev) ✗ python 1.pyf    哈哈，这是f    :param x:    :return:

以上是别的教程写的解决方案，突然对这个functools.wraps()有点兴趣，它到底是如何做到的呢？

functools.warp方法就是把被wrapped的函数的属性传递给wrapper的函数。

WRAPPER_ASSIGNMENTS = ('__module__', '__name__', '__doc__')WRAPPER_UPDATES = ('__dict__',)def update_wrapper(wrapper,                   wrapped,                   assigned = WRAPPER_ASSIGNMENTS,                   updated = WRAPPER_UPDATES):    """Update a wrapper function to look like the wrapped function       wrapper is the function to be updated       wrapped is the original function       assigned is a tuple naming the attributes assigned directly       from the wrapped function to the wrapper function (defaults to       functools.WRAPPER_ASSIGNMENTS)       updated is a tuple naming the attributes of the wrapper that       are updated with the corresponding attribute from the wrapped       function (defaults to functools.WRAPPER_UPDATES)    """    for attr in assigned:        setattr(wrapper, attr, getattr(wrapped, attr))    for attr in updated:        getattr(wrapper, attr).update(getattr(wrapped, attr, {}))    # Return the wrapper so this can be used as a decorator via partial()    return wrapperdef wraps(wrapped,          assigned = WRAPPER_ASSIGNMENTS,          updated = WRAPPER_UPDATES):    """Decorator factory to apply update_wrapper() to a wrapper function       Returns a decorator that invokes update_wrapper() with the decorated       function as the wrapper argument and the arguments to wraps() as the       remaining arguments. Default arguments are as for update_wrapper().       This is a convenience function to simplify applying partial() to       update_wrapper().    """    return partial(update_wrapper, wrapped=wrapped,                   assigned=assigned, updated=updated)

这个乍一看不是很理解，这个怎么和普通的装饰器不大一样呀。普通的长得像：

def out_wrapper(func):    def in_wrapper(*args, **kwargs):        print ‘in_wrapper’        return func(*args, **kwargs)    return in_wrapper

不过剥丝抽茧，先去除注释：

WRAPPER_ASSIGNMENTS = ('__module__', '__name__', '__doc__')WRAPPER_UPDATES = ('__dict__',)def update_wrapper(wrapper, wrapped, assigned = WRAPPER_ASSIGNMENTS, updated = WRAPPER_UPDATES):    for attr in assigned:        setattr(wrapper, attr, getattr(wrapped, attr))    for attr in updated:        getattr(wrapper, attr).update(getattr(wrapped, attr, {}))    return wrapperdef wraps(wrapped, assigned = WRAPPER_ASSIGNMENTS, updated = WRAPPER_UPDATES):    return partial(update_wrapper, wrapped=wrapped, assigned=assigned, updated=updated)

嗯，看起来简单清爽多了。考虑到偏函数，上面代码可以转成：

WRAPPER_ASSIGNMENTS = ('__module__', '__name__', '__doc__')WRAPPER_UPDATES = ('__dict__',)def update_wrapper(wrapper, wrapped, assigned = WRAPPER_ASSIGNMENTS, updated = WRAPPER_UPDATES):    for attr in assigned:        setattr(wrapper, attr, getattr(wrapped, attr))    for attr in updated:        getattr(wrapper, attr).update(getattr(wrapped, attr, {}))    return wrapperdef wraps(wrapped, assigned = WRAPPER_ASSIGNMENTS, updated = WRAPPER_UPDATES):    def in_wrapper(wrapper, wrapped=wrapped, assigned = WRAPPER_ASSIGNMENTS, updated = WRAPPER_UPDATES):        return update_wrapper(wrapper, wrapped=wrapped, assigned = WRAPPER_ASSIGNMENTS,updated = WRAPPER_UPDATES)    return in_wrapper

一般带参数的装饰器，是三层的，类似：

def wrapper(x):    def out_wrapper(func):        def in_wrapper(*args, **kwargs):            print ‘in_wrapper’            return func(*args, **kwargs)        return in_wrapper    return out_wrapper

虽然长得不一样，但思路就是返回一个能包装函数的函数变量，这帮作者写代码真是骚。

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