import collections

class CustomUserDict(collections.UserDict):

    def __missing__(self, key):
        if isinstance(key, str):
            raise KeyError(key)
        return self[str(key)]

    def __contains__(self, key):
        return str(key) in self.data

    def __setitem__(self, key, item):
        self.data[str(key)] = item


if __name__ == '__main__':

    d = CustomUserDict({1:"one", 2:"two", 3:"three"})

    print(d[1])
    print(d['1'])
    print(d[4]) # raises KeyError