class Mapping:
    def __init__(self, iterable):
        self.items_list = []
        self.__update(iterable)

    def __update(self, iterable):
        for item in iterable:
            self.items_list.append(item)

class MappingSubclass(Mapping):

    def __update(self, keys, values):
        # provides new signature for __update()
        # but does not break __init__()
        for item in zip(keys, values):
            self.items_list.append(item)

if __name__ == '__main__':
    m = Mapping([1,2,3])
    n = MappingSubclass([3,4,5])