what is the difference between functools.wraps and update_wrapper

Question

I am not able to find what is the difference between these two python functions.

functools.wraps and update_wrapper

Can some give me

Answer 1

Normally you will just use wraps, which wraps *update_wrapper* . More:

• partial ing a function = create a new function with some arguments bound to values

• wraps partials *update_wrapper* with wrapped, thus creating a decorator for wrapper

• *update_wrapper* 's purpose is to copy certain attributes (not arguments) from wrapped to wrapper. On default these are:

WRAPPER_ASSIGNMENTS = ('__module__', '__name__', '__qualname__', '__doc__',
                       '__annotations__')
WRAPPER_UPDATES = ('__dict__',)

A useful example:

try:
    from itertools import izip_longest as zip_longest
except:
    from itertools import zip_longest
from collections import Iterable
from functools import wraps

def ziplongest(*args):
    '''zip_longest with last element as filler
    >>> args=([9],[2,3],1)
    >>> [t for t in ziplongest(*args)]
    [(9, 2, 1), (9, 3, 1)]

    '''
    iterable = lambda a:(a if isinstance(a,Iterable) else [a])
    _args = [iterable(a) for a in args]
    withnone = zip_longest(*_args)
    for e in withnone:
        yield tuple((en or _args[i][-1]) for i,en in enumerate(e))

def listable(f):
    '''apply f to list members
    >>> @listable
    ... def mul(a,b):
    ...     'returns a*b'
    ...     return a*b
    >>> mul(2,[3,9])
    [6, 18]
    >>> mul.__doc__
    'returns a*b'

    '''
    @wraps(f)#without this e.g __doc__ would get hidden
    def to_elems(*args,**kwargs):
        if any(isinstance(x,list) for x in args):
            return [f(*a,**kwargs) for a in ziplongest(*args)]
        else:
            return f(*args,**kwargs)
    return to_elems

Answer 2

functools.wraps is equivalent to:

def wraps(wrapped, assigned=WRAPPER_ASSIGNMENTS, updated=WRAPPER_UPDATES):
    def decorator(wrapper):
        return update_wrapper(wrapper, wrapped=wrapped, ...)
    return decorator

It's actually implemented using partial instead of an inner function, but the effect is the same.

The purpose is to allow using it as a decorator:

 @wraps(f)
 def g():
     ...

is equivalent to:

def g():
    ...
g = update_wrapper(g, f)