笔记-python-functool-@wraps
1. wraps
经常看到@wraps装饰器，查阅文档学习一下
在了解它之前，先了解一下partial和updata_wrapper这两个前置技能，因为在wraps中用到了。
1.1. partial
偏函数
源代码：
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
用于构造一个新了函数，但新的函数包括了原函数的参数，目前看来多用于简化代码。
def u(a,b,*args):
pass
u1 = partial(u,4,5,7)
print('u:{}\nu1:{}\nu1.func{}\nu1.args{}'.format(u,u1,u1.func,u1.args))
def ad(*args):
return args[0] + args[1]
ad2 = partial(ad,3,4,5)
输出：
u:<function u at 0x000000CF4FA02E18>
u1:functools.partial(<function u at 0x000000CF4FA02E18>, 4, 5, 7)
u1.func<function u at 0x000000CF4FA02E18>
u1.args(4, 5, 7)
>>>
1.2. update_wrapper
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
简单来说，就是把wrapper的相关属性改成和wrapped相同的。返回wrapper
1.3. wraps
回到wraps
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)
核心就一句，实际就是一个修饰器版的update_wrapper，将被修饰的函数（wrapped）
注意这里的写法，wrapped=wrapped,对偏函数而言，这样写的含义与wrapped不一样。
1.4. 总结
wraps装饰器的作用就是更改函数名称和属性。
当使用装饰器装饰一个函数时，函数本身就已经是一个新的函数；即函数名称或属性产生了变化。所以在装饰器的编写中建议加入wraps确保被装饰的函数不会因装饰器带来异常情况。