Yet another way to create a singleton in python

singleton.py

#############################
# descriptor version
# (only one class is supported currently)
#############################

class _Singleton:

    def __set_name__(self, owner, name):
        self.name = name
        self.instance_created = False

    def __set__(self, instance, value):
        if not self.instance_created:
            self.instance_created = True
            instance.__instance = value
        else:
            raise AttributeError(
                f'{instance.__name__} should not be'
                f' created more than once.'
                )

    def __get__(self, instance, owner):
        return instance.__instance


class Singleton(type):

    _instance = _Singleton()

    def __call__(cls, *args, **kwargs):
        if '_instance' not in cls.__dict__:
            cls._instance = super().__call__(*args, **kwargs)
            return cls._instance
        else:
            raise AttributeError(
                f'{cls.__name__} should not be created more than once.'
                )

    @property
    def inst(cls):
        return cls._instance


class Foo(metaclass=Singleton):
    pass

#############################
# dict version
#############################
class Singleton(type):

    _instance = {}

    def __call__(cls, *args, **kwargs):
        if cls.__name__ not in cls._instance:
            cls._instance[cls.__name__] = super().__call__(*args, **kwargs)
            return cls._instance[cls.__name__]
        else:
            raise AttributeError(
                f'{cls.__name__} should not be created more than once.'
                )

    @property
    def inst(cls):
        return cls._instance[cls.__name__]