Python 备忘录模式讲解和代码示例
备忘录是一种行为设计模式， 允许生成对象状态的快照并在以后将其还原。
备忘录不会影响它所处理的对象的内部结构， 也不会影响快照中保存的数据。
复杂度：
流行度：
使用示例： 备忘录的基本功能可用序列化来实现， 这在 Python 语言中很常见。 尽管备忘录不是生成对象状态快照的唯一或最有效的方法， 但它能在保护原始对象的结构不暴露给其他对象的情况下保存对象状态的备份。
概念示例
本例说明了备忘录设计模式的结构并重点回答了下面的问题：
- 它由哪些类组成？
- 这些类扮演了哪些角色？
- 模式中的各个元素会以何种方式相互关联？
main.py: 概念示例
from __future__ import annotations
from abc import ABC, abstractmethod
from datetime import datetime
from random import sample
from string import ascii_letters
class Originator:
"""
The Originator holds some important state that may change over time. It also
defines a method for saving the state inside a memento and another method
for restoring the state from it.
"""
_state = None
"""
For the sake of simplicity, the originator's state is stored inside a single
variable.
"""
def __init__(self, state: str) -> None:
self._state = state
print(f"Originator: My initial state is: {self._state}")
def do_something(self) -> None:
"""
The Originator's business logic may affect its internal state.
Therefore, the client should backup the state before launching methods
of the business logic via the save() method.
"""
print("Originator: I'm doing something important.")
self._state = self._generate_random_string(30)
print(f"Originator: and my state has changed to: {self._state}")
@staticmethod
def _generate_random_string(length: int = 10) -> str:
return "".join(sample(ascii_letters, length))
def save(self) -> Memento:
"""
Saves the current state inside a memento.
"""
return ConcreteMemento(self._state)
def restore(self, memento: Memento) -> None:
"""
Restores the Originator's state from a memento object.
"""
self._state = memento.get_state()
print(f"Originator: My state has changed to: {self._state}")
class Memento(ABC):
"""
The Memento interface provides a way to retrieve the memento's metadata,
such as creation date or name. However, it doesn't expose the Originator's
state.
"""
@abstractmethod
def get_name(self) -> str:
pass
@abstractmethod
def get_date(self) -> str:
pass
class ConcreteMemento(Memento):
def __init__(self, state: str) -> None:
self._state = state
self._date = str(datetime.now())[:19]
def get_state(self) -> str:
"""
The Originator uses this method when restoring its state.
"""
return self._state
def get_name(self) -> str:
"""
The rest of the methods are used by the Caretaker to display metadata.
"""
return f"{self._date} / ({self._state[0:9]}...)"
def get_date(self) -> str:
return self._date
class Caretaker:
"""
The Caretaker doesn't depend on the Concrete Memento class. Therefore, it
doesn't have access to the originator's state, stored inside the memento. It
works with all mementos via the base Memento interface.
"""
def __init__(self, originator: Originator) -> None:
self._mementos = []
self._originator = originator
def backup(self) -> None:
print("\nCaretaker: Saving Originator's state...")
self._mementos.append(self._originator.save())
def undo(self) -> None:
if not len(self._mementos):
return
memento = self._mementos.pop()
print(f"Caretaker: Restoring state to: {memento.get_name()}")
try:
self._originator.restore(memento)
except Exception:
self.undo()
def show_history(self) -> None:
print("Caretaker: Here's the list of mementos:")
for memento in self._mementos:
print(memento.get_name())
if __name__ == "__main__":
originator = Originator("Super-duper-super-puper-super.")
caretaker = Caretaker(originator)
caretaker.backup()
originator.do_something()
caretaker.backup()
originator.do_something()
caretaker.backup()
originator.do_something()
print()
caretaker.show_history()
print("\nClient: Now, let's rollback!\n")
caretaker.undo()
print("\nClient: Once more!\n")
caretaker.undo()
Output.txt: 执行结果
Originator: My initial state is: Super-duper-super-puper-super.
Caretaker: Saving Originator's state...
Originator: I'm doing something important.
Originator: and my state has changed to: wQAehHYOqVSlpEXjyIcgobrxsZUnat
Caretaker: Saving Originator's state...
Originator: I'm doing something important.
Originator: and my state has changed to: lHxNORKcsgMWYnJqoXjVCbQLEIeiSp
Caretaker: Saving Originator's state...
Originator: I'm doing something important.
Originator: and my state has changed to: cvIYsRilNOtwynaKdEZpDCQkFAXVMf
Caretaker: Here's the list of mementos:
2019-01-26 21:11:24 / (Super-dup...)
2019-01-26 21:11:24 / (wQAehHYOq...)
2019-01-26 21:11:24 / (lHxNORKcs...)
Client: Now, let's rollback!
Caretaker: Restoring state to: 2019-01-26 21:11:24 / (lHxNORKcs...)
Originator: My state has changed to: lHxNORKcsgMWYnJqoXjVCbQLEIeiSp
Client: Once more!
Caretaker: Restoring state to: 2019-01-26 21:11:24 / (wQAehHYOq...)
Originator: My state has changed to: wQAehHYOqVSlpEXjyIcgobrxsZUnat
