web备忘录模式中的类怎么定义

class Originator:    def __init__(self):        self.a= 0        self.b= 0        self.c= 0    defprint(self):        print('a=', self.a, ',b=', self.b,',c=', self.c)    def set_memento(self, m):        self.__dict__ = m    def create_memento(self):        returncopy.copy(self.__dict__)

if __name__ == "__main__":    c = []    o = Originator()    o.print()    c.append(o.create_memento())    o.a = 1    o.print()    c.append(o.create_memento())    o.b = 2    o.print()    c.append(o.create_memento())    o.c = 3    o.print()    print('start undo')    o.print()    while len(c):        o.set_memento(c.pop())        o.print()

a= 0 ,b= 0 ,c= 0a= 1 ,b= 0 ,c= 0a= 1 ,b= 2 ,c= 0a= 1 ,b= 2 ,c= 3start undoa= 1 ,b= 2 ,c= 3a= 1 ,b= 2 ,c= 0a= 1 ,b= 0 ,c= 0a= 0 ,b= 0 ,c= 0