# works in Python 2 & 3
class _Singleton(type):
    """ A metaclass that creates a Singleton base class when called. """

    _instances = {}

    def __call__(cls, *args, **kwargs):
        if cls not in cls._instances:
            cls._instances[cls] = super(_Singleton, cls).__call__(
                *args, **kwargs
            )
        return cls._instances[cls]


class Singleton(_Singleton("SingletonMeta", (object,), {})):
    pass
