[python 基础]python装饰器(一)添加functools获取原函数信息以及functools.partial分析

python装饰器学习的时候有两点需要注意一下

1,被装饰器装饰的函数取其func.__name__和func.func_doc的时候得到的不是被修饰函数的相关信息而是装饰器wrapper函数的docstring和名字

对此我们使用functools这个模块添加一行函数即可

@functools.wraps(f)def check_id_admin(f):

'''检查是否为admin'''
    # 使得__name__和func_doc能够获得函数原有的docstring和函数名而不是装饰器相关的
    @functools.wraps(f)
    def wrapper(*args,**kwargs):if kwargs.get("username")!="admin":
            raise Exception("this user is not allowed to get food!")
        return f(*args,**kwargs)
    return wrapper

@check_id_admin
def get_food(username,password,food="chocolate"):
    '''get food'''
    return "%s get food: %s"%(username,food)

def main():
    print(get_food.__name__)
    print(get_food.func_doc)

#输出结果:

  get_food
  get food

　对比不使用@functools.wrap(f)

　wrapper

　none　

2.查看functools.wraps()源码

RAPPER_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:
　　　　　#在这里获取原函数wrapped的attr赋值给value
            value = getattr(wrapped, attr)
        except AttributeError:
            pass
        else:
            setattr(wrapper, attr, value)
    for attr in updated:
　　　　　#用wrapped获取的值更新wrapper中的attr
        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

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)

 

从上面代码中可以看出wraps这个函数是通过partial和update_wrapper来实现的

(1)update_wrapper函数

update_wrapper做的工作很简单，就是用参数wrapped表示的函数对象（例如：square）的一些属性()如：__name__、 __doc__）覆盖参数wrapper表示的函数对象的这些相应属性.

即先保存被更新函数信息再更新到新函数中再返回给调用者。

(2)partial函数

简单总结functools.partial的作用就是，把一个函数的某些参数给固定住（也就是设置默认值），返回一个新的函数，调用这个新函数会更简单。上代码:

from functools import partial

def add(a,b,c):
    return a+b+c
#固定bc两个参数->add_one仅需要一个参数a
add_one = partial(add,b=1,c=3)
#固定a参数->add_two不需要参数运算
add_two = partial(add_one,a=2)
print(add_one(a=3))
print(add_two())

因此

#返回一个参数给定的update_wrapper函数
return partial(update_wrapper, wrapped=wrapped,assigned=assigned, updated=updated)