模版如下：

class Singleton:
    __instance = None
    def __new__(cls, *args):
        if cls.__instance is None:
            cls.__instance = object.__new__(cls, *args)
        return cls.__instance

原文链接：
https://medium.com/techtofreedom/3-levels-of-understanding-the-singleton-pattern-in-python-4bf429a10438?sk=392159676dc87218a1b7b8c6830bc277