利用Python中的元类实现单例模式

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class Singleton(type):
    def __init__(self, *args, **kwargs):
        self.__instance = None
        # 如果是Python2
        # super(Singleton,self).__init__(*args, **kwargs)
        super().__init__(*args, **kwargs)
    def __call__(self, *args, **kwargs):
        if self.__instance is None:
            self.__instance = super().__call__(*args, **kwargs)
            return self.__instance
        else:
            return self.__instance
class Spam(metaclass=Singleton):
    # 如果是Python2
    # __metaclass__=Singleton
    def __init__(self):
        print('Creating Spam')
if __name__ == '__main__':
    a = Spam()
    b = Spam()
    print(a is b)