关于装饰器中 @wraps 的部分实现原理

我们在定义装饰器时都会使用 @wraps 让我们被装饰的函数保持自己的一些私有属性


    def decorator(f):
        @wraps(f)
        def decorated(*args, **kwargs):
            print('ok')
            return f(*args, **kwargs)

        return decorated

那么@wraps 是如何实现的呢，先看一下wraps 的代码：

WRAPPER_ASSIGNMENTS = ('__module__', '__name__', '__qualname__', '__doc__',
                       '__annotations__')
WRAPPER_UPDATES = ('__dict__',)
def 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)

这里 @wraps 返回的也是partial(update_wrapper, wrapped=wrapped assigned=assigned, updated=updated)，这里partial 是如何实现的，本人能力有限不作讨论，只要知道partial(update_wrapper, wrapped=wrapped assigned=assigned, updated=updated)(function)与 update_wrapper(function,wrapped=wrapped assigned=assigned, updated=updated)是一样的就可以了，接着看update_wrapper

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:
        try:
            value = getattr(wrapped, attr)
        except AttributeError:
            pass
        else:
            setattr(wrapper, attr, value)
    for attr in updated:
        getattr(wrapper, attr).update(getattr(wrapped, attr, {}))
    # Issue #17482: set __wrapped__ last so we don't inadvertently copy it
    # from the wrapped function when updating __dict__
    wrapper.__wrapped__ = wrapped
    # Return the wrapper so this can be used as a decorator via partial()
    return wrapper

我们按照最开始写的装饰器的代码把真实参数传进去

update_wrapper(wrapper, f, assigned = ('__module__', '__name__', '__qualname__', '__doc__', '__annotations__'), updated = ('__dict__',))

有了真实的参数后，我们再将它带入最开始的代码


    def decorator(f):
        @update_wrapper(wrapper, f, assigned = ('__module__', '__name__', '__qualname__', '__doc__', '__annotations__'), updated = ('__dict__',)) 
        def decorated(*args, **kwargs):
            print('ok')
            return f(*args, **kwargs)

        return decorated

这里的wrapper 就是我们要装饰的下面的函数decorated ，由于partial 的作用，上面的代码可以转换成如下：


    def decorator(f):
        update_wrapper(decorated, f, assigned = ('__module__', '__name__', '__qualname__', '__doc__', '__annotations__'), updated = ('__dict__',))

参照上面update_wrapper的实现我们可以知道，就是将f 的属性绑定到decorated 上去，因为我们使用装饰器时：

@decorator
def login():
    return 'hello'

相当于

login=decorator(login)=decorated

可以看到被装饰器修饰后的函数login已经变成了函数decorated,参考上面，可以知道，由于@wraps的存在，已经把f 即（函数login）的属性绑定到decorated 上了，此时调用被装饰后的函数login的属性得到的和装饰前的属性是一致的。

若在定义装饰器时没有使用@wraps

#from functools import wraps


def decorator(f):
#    @wraps(f)
    def decorated(*args, **kwargs):
        print('ok')
        return f(*args, **kwargs)

    return decorated


@decorator
def test():
    return 'ok'

print(test.__name__)

得到的值是 decorated 而使用了@wraps的值是test
本人菜鸟一个，如果有错欢迎指出。

关于装饰器中 @wraps 的部分实现原理

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