单例模式

类方法和__new__实现的单例模式
# 单例模式
# singleton pattern


class Demo:
    __instance = None

    @classmethod
    def singleton(cls):
        if not cls.__instance:
            cls.__instance = cls()
        return cls.__instance


class Demo1:
    __instance = None

    def __new__(cls, *args, **kwargs):
        if not cls.__instance:
            cls.__instance = object.__new__(cls)
        return cls.__instance