单例模式，多次实例化只返回同一个类实例。

class SimpleOne(object):
    __one = None

    def __init__(self):
        super().__init__()

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


if __name__ == "__main__":
    id_1 = id(SimpleOne())
    id_2 = id(SimpleOne())
    print("id_1 equal id_2", id_1 == id_2)