unsafe_singleton.py

class SingletonMeta(type):
    _instance = None
    _args = None
    _kwargs = None

    def __call__(cls, *args, **kwargs):
        if not cls._instance:
            cls._args = args
            cls._kwargs = kwargs
            cls._instance = super(SingletonMeta, cls).__call__(*args, **kwargs)
        else:
            if cls._args != args or cls._kwargs != kwargs:
                raise ValueError

        return cls._instance


class Singleton(metaclass=SingletonMeta):
    pass