单例模式

class Single:
    _instance = None

    def __new__(cls, *args, **kwargs):
        if not cls._instance:
            cls._instance = super(Single, cls).__new__(cls, *args, **kwargs)
        return cls._instance


class Demon(Single):

    def foo(self):
        print("Hi")


d1 = Demon()
d2 = Demon()
print(id(d1))　　# 2110545800776
print(id(d2))　　# 2110545800776