Исходный код vk_maria.mixins

class _Singleton(type):
    _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]

    def get_current(cls):
        return cls._instances[cls]


class Singleton(metaclass=_Singleton):
    pass