状态模式，就像某一个人在儿童、青年、老年所展现的状态是不同的。

class Obj(object):
    def __init__(self, status):
        self.status = status

    def do(self):
        which_do = {"low": self.low_do,
                    "middle": self.middle_do,
                    "high": self.high_do
                    }
        which_do[self.status]()

    def low_do(self):
        print("low_do")

    def middle_do(self):
        print("middle_do")

    def high_do(self):
        print("high_do")


def main():
    obj = Obj('low')
    obj.do()
    obj.status = 'middle'
    obj.do()


if __name__ == '__main__':
    main()