python functools 中 partial 和 wraps 的用处

function_partial_example.py

#!/bin/env python
# -*- coding: utf-8 -*-

""" partial 实例.

看上面的源代码, 可以看出 partial 接受一个函数和若干参数, 返回一个新的函数，新的函数已经包含了之前的函数和参数,
而新函数执行时引入的新参数会和之前的参数相加, 真正执行的函数还是之前的函数.

所以, partial 的作用是动态绑定参数, 第一次用 partial 生成新函数, 第二次继续绑定参数并执行.

"""

from function import partial


basetwo = partial(int, base=2)
basetwo.__doc__ = 'Convert base 2 string to an int.'
print basetwo('10010')

function_partial_implement.py

""" python 的 function.partial 是 C实现的, 这里是一个 Python 实现, 效果相同, 放在这里理解.

"""

def partial(func, *args, **keywords):
    # print func, args, keywords
    def newfunc(*fargs, **fkeywords):
        newkeywords = keywords.copy()
        newkeywords.update(fkeywords)
        # print (args + fargs), newkeywords
        return func(*(args + fargs), **newkeywords)
    newfunc.func = func
    newfunc.args = args
    newfunc.keywords = keywords
    #print newfunc.func, newfunc.args, newfunc.keywords
    return newfunc

functools_nowraps.py

#!/bin/env python

"""
output:

Calling decorated function
Called example function
wrapper
None

"""

from functools import wraps


def my_decorator(f):
    #@wraps(f)
    def wrapper(*args, **kwds):
        print 'Calling decorated function'
        return f(*args, **kwds)
    return wrapper


@my_decorator
def example():
    """Docstring"""
    print 'Called example function'


example()
print example.__name__
print example.__doc__

functools_wraps.py

#!/bin/env python


"""
output:

Calling decorated function
Called example function
example
Docstring

"""

from functools import wraps


def my_decorator(f):
    @wraps(f)
    def wrapper(*args, **kwds):
        print 'Calling decorated function'
        return f(*args, **kwds)
    return wrapper


@my_decorator
def example():
    """Docstring"""
    print 'Called example function'


example()
print example.__name__
print example.__doc__

wraps_source.py

""" wraps 源代码.

用于理解 wraps 是如何工作的.

"""

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
    
    
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)