signature functools wraps example

from inspect import signature
from functools import wraps

def dec(func):
    def wrapper(*args, **kwargs):
        return func(*args, **kwargs)
    return wrapper

def dec2(func):
    @wraps(func)
    def wrapper(*args, **kwargs):
        return func(*args, **kwargs)
    return wrapper

def foo(a: int, b):
    pass

print(signature(dec(foo)))
print(signature(dec2(foo)))

prints:

(*args, **kwargs)
(a:int, b)

As per python 3.7: functools.wraps preserves signature but only in appearance because help and signature follow the __wrapped__ attribute ; it does not raise correct errors in case of misuse. See this answer, you can use makefun.wraps to get a fully signature-preserving behaviour like in decorator. A very interesting side effect is that all arguments are redirected into the kwargs variable when possible (when not varpositional), which is really helpful for the wrapper implementation.

from inspect import signature
from functools import wraps

def dec2(func):
    @wraps(func)
    def wrapper(*args, **kwargs):
        print("success!")
        return func(*args, **kwargs)
    return wrapper

from makefun import wraps

def dec3(func):
    @wraps(func)
    def wrapper(*args, **kwargs):
        print("success!")
        return func(*args, **kwargs)
    return wrapper

def foo(a: int, b):
    pass

f2 = dec2(foo)
print(signature(f2))
f2(1)  # raises TypeError but... prints "success!"

f3 = dec3(foo)
print(signature(f3))
f3(1)  # raises TypeError and *does not* print success