Python装饰器20-update_wrapper

使用update_wrapper改变装饰后函数的属性

Python还提供定制化更强的函数update_wrapper来改变装饰后的函数属性，使用示例。

from functools import update_wrapper

def add_dec(f):
    def wrapper(*args, **kwargs):
        'wrapper'
        print('new wrapper')
        return f(*args, **kwargs)
    return update_wrapper(wrapper, f)

@add_dec
def add(x, y):
    'original add'
    return x + y

print(add(3, 3))
print(add)
print(add.__name__)
print(add.__doc__)

new wrapper
6

add
original add

看样子效果跟使用@wraps(f)是一样的

update_wrapper函数源码

参看源码，正如之前的理解。

WRAPPER_ASSIGNMENTS = ('__module__', '__name__', '__qualname__', '__doc__',
                       '__annotations__')
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:
        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

默认拷贝了'__module__', '__name__', '__qualname__', '__doc__','__annotations__'属性。

结论

熟悉update_wrapper有助于理解@wraps的实现机制。