copy-signature.py

import types
import inspect


def impl(x, *, y, z=None):
    print(x, y, z, "impl")


def dispatch(x, y, z):
    print(x, y, z, "dispatch")


def copy_signature(source, dest):
    dest_code: types.CodeType = dest.__code__
    source_code: types.CodeType = source.__code__

    assert dest_code.co_kwonlyargcount == 0
    assert dest_code.co_posonlyargcount == 0
    assert dest_code.co_varnames == source_code.co_varnames

    new_dest_code = dest_code.replace(
        co_argcount=source_code.co_argcount,
        co_posonlyargcount=source_code.co_posonlyargcount,
        co_kwonlyargcount=source_code.co_kwonlyargcount,
    )

    new_dest = types.FunctionType(
        new_dest_code,
        source.__globals__,
        dest.__name__,
        source.__defaults__,
        source.__closure__,
    )
    new_dest.__kwdefaults__ = source.__kwdefaults__
    return new_dest


dispatch_like_impl = copy_signature(impl, dispatch)

impl(1, y=2)
dispatch_like_impl(1, y=2)