How to enforce method signature for child classes?

Question

Languages like C#, Java has method overloads, which means if child class does not implement the method with exact signature will not overwrite the parent method.

How

Answer 1

Update of the accepted answer to work with python 3.5.

import inspect
from types import FunctionType

class BadSignatureException(Exception):
    pass


class SignatureCheckerMeta(type):
    def __new__(cls, name, baseClasses, d):
        #For each method in d, check to see if any base class already
        #defined a method with that name. If so, make sure the
        #signatures are the same.
        for methodName in d:
            f = d[methodName]

            if not isinstance(f, FunctionType):
                continue
            for baseClass in baseClasses:
                try:
                    fBase = getattr(baseClass, methodName)
                    if not inspect.getargspec(f) == inspect.getargspec(fBase):
                        raise BadSignatureException(str(methodName))
                except AttributeError:
                    #This method was not defined in this base class,
                    #So just go to the next base class.
                    continue

        return type(name, baseClasses, d)


def main():
    class A(object):
        def foo(self, x):
            pass

    try:
        class B(A, metaclass=SignatureCheckerMeta):
            def foo(self):
                """This override shouldn't work because the signature is wrong"""
                pass
    except BadSignatureException:
        print("Class B can't be constructed because of a bad method signature")
        print("This is as it should be :)")

    try:
        class C(A):
            __metaclass__ = SignatureCheckerMeta
            def foo(self, x):
                """This is ok because the signature matches A.foo"""
                pass
    except BadSignatureException:
        print("Class C couldn't be constructed. Something went wrong")


if __name__ == "__main__":
    main()