Python 关于functools模块的wraps装饰器用途

测试环境：Python3.6.2 + win10 +  Pycharm2017.3

---

 装饰器之functools模块的wraps的用途：

首先我们先写一个装饰器

# 探索functools模块wraps装饰器的用途
from functools import wraps


def trace(func):
    """ 装饰器 """

    # @wraps(func)
    def callf(*args, **kwargs):
        """ A wrapper function """
        print("Calling function:{}".format(func.__name__))  # Calling function:foo
        res = func(*args, **kwargs)
        print("Return value:{}".format(res))  # Return value:9
        return res

    return callf


@trace
def foo(x):
    """ 返回给定数字的平方 """
    return x * x


if __name__ == '__main__':
    print(foo(3))  # 9
    print(foo.__doc__)
    help(foo)
    print(foo.__name__)
    # print(foo.__globals__)
    t = trace(foo)
    print(t)
打印结果：
Calling function:foo
Return value:9
9
 A wrapper function 
Help on function callf in module __main__:

callf(*args, **kwargs)
    A wrapper function

callf
<function trace.<locals>.callf at 0x0000022F744D8730>

上面的装饰器例子等价于：trace(foo(3)),只是在使用装饰器时，我们不用再手动调用装饰器函数；

如果把这段代码提供给其他人调用， 他可能会想看下foo函数的帮助信息时：

>>>from xxx import foo
>>>help(foo)    # print(foo__doc__)
Help on function callf in module __main__:

callf(*args, **kwargs)
    A wrapper function

这里，他可能会感到迷惑，继续敲：

>>> print(foo.__name__)
callf

最后， 他可能会看源码，找问题原因，我们知道Python中的对象都是"第一类"的，所以，trace函数会返回一个callf闭包函数，连带callf的上下文环境一并返回，所以，可以解释我们执行help(foo)的到结果了

那么，我们如果才能得到我们想要的foo的帮助信息呢，这里就要用到了functools的wraps了。

# 探索functools模块wraps装饰器的用途
from functools import wraps


def trace(func):
    """ 装饰器 """

    @wraps(func)
    def callf(*args, **kwargs):
        """ A wrapper function """
        print("Calling function:{}".format(func.__name__))  # Calling function:foo
        res = func(*args, **kwargs)
        print("Return value:{}".format(res))  # Return value:9
        return res

    return callf


@trace
def foo(x):
    """ 返回给定数字的平方 """
    return x * x


if __name__ == '__main__':
    print(foo(3))  # 9
    print(foo.__doc__)
    help(foo)
    print(foo.__name__)
    # print(foo.__globals__)
    t = trace(foo)
    print(t)

至于wraps的原理，通过下面部分源码，可以自行研究，在此就不予展开扩展了

# 有关wraps的源码，有兴趣的可以自行研究下


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

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)








class partial:
    """New function with partial application of the given arguments
    and keywords.
    """

    __slots__ = "func", "args", "keywords", "__dict__", "__weakref__"

    def __new__(*args, **keywords):
        if not args:
            raise TypeError("descriptor '__new__' of partial needs an argument")
        if len(args) < 2:
            raise TypeError("type 'partial' takes at least one argument")
        cls, func, *args = args
        if not callable(func):
            raise TypeError("the first argument must be callable")
        args = tuple(args)

        if hasattr(func, "func"):
            args = func.args + args
            tmpkw = func.keywords.copy()
            tmpkw.update(keywords)
            keywords = tmpkw
            del tmpkw
            func = func.func

        self = super(partial, cls).__new__(cls)

        self.func = func
        self.args = args
        self.keywords = keywords
        return self

    def __call__(*args, **keywords):
        if not args:
            raise TypeError("descriptor '__call__' of partial needs an argument")
        self, *args = args
        newkeywords = self.keywords.copy()
        newkeywords.update(keywords)
        return self.func(*self.args, *args, **newkeywords)

    @recursive_repr()
    def __repr__(self):
        qualname = type(self).__qualname__
        args = [repr(self.func)]
        args.extend(repr(x) for x in self.args)
        args.extend(f"{k}={v!r}" for (k, v) in self.keywords.items())
        if type(self).__module__ == "functools":
            return f"functools.{qualname}({', '.join(args)})"
        return f"{qualname}({', '.join(args)})"

    def __reduce__(self):
        return type(self), (self.func,), (self.func, self.args,
               self.keywords or None, self.__dict__ or None)

    def __setstate__(self, state):
        if not isinstance(state, tuple):
            raise TypeError("argument to __setstate__ must be a tuple")
        if len(state) != 4:
            raise TypeError(f"expected 4 items in state, got {len(state)}")
        func, args, kwds, namespace = state
        if (not callable(func) or not isinstance(args, tuple) or
           (kwds is not None and not isinstance(kwds, dict)) or
           (namespace is not None and not isinstance(namespace, dict))):
            raise TypeError("invalid partial state")

        args = tuple(args) # just in case it's a subclass
        if kwds is None:
            kwds = {}
        elif type(kwds) is not dict: # XXX does it need to be *exactly* dict?
            kwds = dict(kwds)
        if namespace is None:
            namespace = {}

        self.__dict__ = namespace
        self.func = func
        self.args = args
        self.keywords = kwds

只要是知道，wraps是通过partial和update_wrapper来帮我们实现想要的结果的

---

摘自：

　　https://www.cnblogs.com/myd7349/p/how_to_use_wraps_of_functools.html    # 关于functools模块的wraps装饰器

---

End