class SingletonMeta(type):

    """ Metaclass that restricts the instantiation of a class to one object """
    _instances = {}

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


class SingleValue(metaclass=SingletonMeta):
    pass