TLDR;

I have a decorator that:

• changes the function signature
• the wrapped function uses some generic type arguments
• Other than the signature I would like to use funtools.wraps to preserve the rest of the information.

Is there any way to achieve that without mypy complaining?

More context

A minimal working example would look like this:

from functools import wraps
from typing import Callable, TypeVar


B = TypeVar('B', bound=str)

def str_as_int_wrapper(func: Callable[[int], int]) -> Callable[[B], B]:
    WRAPPER_ASSIGNMENTS = ('__module__', '__name__', '__qualname__', '__doc__',)
    WRAPPER_UPDATES = ('__dict__', '__annotations__')
    
    @wraps(func, assigned=WRAPPER_ASSIGNMENTS, updated=WRAPPER_UPDATES)
    def _wrapped_func(val: B) -> B:
        num = int(val)
        result = func(num)
        return val.__class__(result)
    
    return _wrapped_func

@str_as_int_wrapper
def add_one(val: int) -> int:
    return val + 1

This seems to work alright, but mypy (version 1.10.0) does not like it. Instead, it complains with

test.py:17: error: Incompatible return value type (got "_Wrapped[[int], int, [Never], Never]", expected "Callable[[B], B]")  [return-value]
test.py:17: note: "_Wrapped[[int], int, [Never], Never].__call__" has type "Callable[[Arg(Never, 'val')], Never]"

If I either remove the @wraps decorator or replace the B type annotations by str, the error disappears.

Question

Am I missing something? Is this some already reported bug or limitation from mypy (couldn’t find anything)? Should it be reported?

Thanks!