functools中@waps的使用
09 March 2015
functools中@waps
使用装饰器的一个副作用就是，装饰之后函数丢失了它本来的__name__,__doc__及__module__属性。
def my_decorator(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__
输出的结果是:
>>>Calling decorated function
>>>Called example function
>>>wrapper
>>>None
使用functools.wraps中@wraps装饰器可以让丢失的函数属性__name__,__doc__,__module__恢复
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__
加入@wraps的输出结果是:
>>>Calling decorated function
>>>Called example function
>>>example
>>>Docstring
@wraps实现原理
上面的例子，函数的调用顺序是:
example() -> my_decorator(example)() ->
wraps(example, assigned = WRAPPER_ASSIGNMENTS, updated = WRAPPER_UPDATES)(wrapper)()
查看wraps的实现代码是：
def wraps(wrapped,
assigned = WRAPPER_ASSIGNMENTS,
updated = WRAPPER_UPDATES):
return partial(update_wrapper,
wrapped=wrapped,
assigned=assigned,
updated=updated)
即:
partial(update_wrapper, wrapped=example, assigned=assigned, updated=updated)(wrapper)()
注意：调用函数partial之后，wrapped=example在函数partial参数keywords中
functools.partial的大概的Python实现是：
def partial(func, *args, **keywords):
def newfunc(*fargs, **fkeywords):
newkeywords = keywords.copy()
newkeywords.update(fkeywords)
return func(*(args + fargs), **newkeywords)
newfunc.func = func
newfunc.args = args
newfunc.keywords = keywords
return newfunc
因此newfunc.func = update_wrapper，newfunc.args = example
函数成为:newfunc(wrapper)，因此newfunc函数最终执行的是update_wrapper(wrapper, wrapped = example,...)
再看update_wrapper的实现.
WRAPPER_ASSIGNMENTS = ('__module__', '__name__', '__doc__')
WRAPPER_UPDATES = ('__dict__',)
def update_wrapper(wrapper,
wrapped,
assigned = WRAPPER_ASSIGNMENTS,
updated = WRAPPER_UPDATES):
for attr in assigned:
setattr(wrapper, attr, getattr(wrapped, attr))
for attr in updated:
getattr(wrapper, attr).update(getattr(wrapped, attr, {}))
return wrapper
partial函数的使用
Python 中有个 int([x[,base]]) 函数，作用是把字符串转换为一个普通的整型。
如果要把所有输入的二进制数转为整型，那么就要这样写 int('11', base=2)。这样写起来貌似不太方便，那么我们就能用 partial 来实现值传递一个参数就能转换二进制数转为整型的方法。
from functools import partial
int2 = partial(int, base=2)
print int2('11') # 3
print int2('101') # 5
附录：完整的实现（没有使用库函数）
def partial_t(func, *args, **keywords):
print "keywords is %s" % keywords
print args
def newfunc(*fargs, **fkeywords):
newkeywords = keywords.copy()
newkeywords.update(fkeywords)
print fargs
print "newkeywords is %s" % newkeywords
return func(*(args + fargs), **newkeywords)
newfunc.func = func
newfunc.args = args
newfunc.keywords = keywords
return newfunc
WRAPPER_ASSIGNMENTS = ('__module__', '__name__', '__doc__')
WRAPPER_UPDATES = ('__dict__',)
def update_wrapper(wrapper,
wrapped,
assigned = WRAPPER_ASSIGNMENTS,
updated = WRAPPER_UPDATES):
for attr in assigned:
setattr(wrapper, attr, getattr(wrapped, attr))
for attr in updated:
getattr(wrapper, attr).update(getattr(wrapped, attr, {}))
return wrapper
def wraps(wrapped,
assigned = WRAPPER_ASSIGNMENTS,
updated = WRAPPER_UPDATES):
return partial_t(update_wrapper, wrapped=wrapped,
assigned=assigned, updated=updated)
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__
输出结果:
keywords is {'wrapped': <function example at 0x0000000002649CF8>, 'assigned': ('__module__', '__name__', '__doc__'), 'updated': ('__dict__',)}
() #args
(<function wrapper at 0x0000000002649DD8>,) #fargs
newkeywords is {'wrapped': <function example at 0x0000000002649CF8>, 'assigned': ('__module__', '__name__', '__doc__'), 'updated': ('__dict__',)}
Calling decorated function
Called example function
example
Docstring