الگوی یادگاری

""" Memento pattern example. """  class Memento(object):     def __init__(self, state):         self._state = state      def get_saved_state(self):         return self._state  class Originator(object):     _state = ""      def set(self, state):         print("Originator: Setting state to", state)         self._state = state      def save_to_memento(self):         print("Originator: Saving to Memento.")         return Memento(self._state)      def restore_from_memento(self, memento):         self._state = memento.get_saved_state()         print("Originator: State after restoring from Memento:", self._state)  saved_states = [] originator = Originator() originator.set("State1") originator.set("State2") saved_states.append(originator.save_to_memento())  originator.set("State3") saved_states.append(originator.save_to_memento())  originator.set("State4")  originator.restore_from_memento(saved_states[0])