策略模式，同一问题有多种不同的解法，即不同策略，一个物体可以动态地对策略进行更换。

class Stragety(object):
    def __init__(self, name):
        self.name = name

    def do(self):
        print(self.name)


class Question(object):
    def __init__(self, question):
        self.question = question
        self.stragety = None

    def set_stragety(self, stragety):
        self.stragety = stragety

    def solve(self):
        self.stragety.do()


def main():
    question = Question('need to solve problem')
    stragety_a = Stragety('a')
    question.set_stragety(stragety_a)
    question.solve()
    stragety_b = Stragety('b')
    question.set_stragety(stragety_b)
    question.solve()


if __name__ == '__main__':
    main()