class Singleton(type):
    def __init__(cls, name, bases, dict):
        super(Singleton, cls).__init__(name, bases, dict)
        cls.instance = None

    def __call__(cls, *args, **kw):
        if cls.instance is None:
            cls.instance = super(Singleton, cls).\
                __call__(*args, **kw)
        return cls.instance


# class Singleton():
#     instance = None

#     def __init__(self):
#         self.__class__.instance = self

#     def get_instance():
#         if self.__class__.instance == None:
#             raise RuntimeError('Server has not been initialized.')
#         else:
#             return self.__class__.instance