python 装饰器的缺点以及解决方法

1.python装饰器的缺点

装饰器可以允许我们在不改变函数或犯方法的调用方式的情况下，添加额外的功能;

如下所示,我们要在中的方法之前增加装饰器check_is_admin,用来判断执行类的方法的用户是否为admin用户;

def check_is_admin(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

class store(object):
    @check_is_admin
    def get_food(self,username,food):
        return self.storage.get(food)

    @check_is_admin
    def put_food(self,username,food):
        self.storage.put(food)

但是，经过装饰器修饰的函数,其func_name和func_doc的属性都会丢失;

如下:

def foobar(username="someone"):
    """Do crzay staff."""
    pass
print foobar.func_doc
print foobar.func_name

执行结果为:
Do crzay staff.
foobar

如果给上述函数增加装饰器呢?

def is_admin(f):
    def wrapper(*args,**kwargs):
        if kwargs.get(*args,**kwargs):
            raise Exception("This user is not allowed to get food")
        return f(*args,**kwargs)
    return wrapper


@is_admin
def foobar(username="someone"):
    """Do crazy staff"""
    pass
print foobar.__doc__
print foobar.__name__

执行结果为:
None
wrapper

update_wapper的源码如下:

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 wrapper

　　

2.解决方案

使用functools的update_wrapper函数可以解决该问题

如下:

def is_admin(f):
    def wrapper(*args,**kwargs):
        if kwargs.get(*args,**kwargs):
            raise Exception("This user is not allowed to get food")
        return f(*args,**kwargs)
    return wrapper

def foobar(username="someone"):
    """Do crazy staff"""
    pass

import functools
foobar = functools.update_wrapper(is_admin,foobar)
print foobar.__name__
print foobar.__doc__

执行结果如下:
foobar
Do crazy staff

但是上述方式手工调用装饰器不太方便,我们在这里使用functools.warps的装饰器

import functools

def is_admin(f):
    @functools.wraps(f)
    def wrapper(*args,**kwargs):
        if kwargs.get(*args,**kwargs):
            raise Exception("This user is not allowed to get food")
        return f(*args,**kwargs)
    return wrapper

@is_admin
def foobar(username="someone"):
    """Do crazy staff"""
    pass
print foobar.__doc__
print foobar.__name__

执行结果为:
Do crazy staff #####
foobar

结论:python的装饰器会导致被修饰函数的__doc__,__name__等属性丢掉,如果要保留函数的这些属性，需要在装饰器函数中添加functools.wrap装饰器;