import copy
import functools
from typing import Callable, TypeVar


T = TypeVar("T", bound=Callable)

def copydefaults(f: T) -> T:
    original_defaults = f.__defaults__

    @functools.wraps(f)
    def newf(*args, **kwargs):
        f.__defaults__ = copy.deepcopy(original_defaults)
        return f(*args, **kwargs)

    return newf


@copydefaults
def f(key, d={}):
    d[key] = 42
    return d

print(f("foo"))  # {'foo': 42}
print(f("bar"))  # {'bar': 42}