备忘录模式和观察者模式 15/30 | Python 主题月

写在前面

本文正在参加「Python主题月」，详情查看活动链接

这个月是 Python 活动月，我决定尝试用 Python 来刷这 30 天的每日一题和随机一题。然后如果周末有精力，我想捣鼓捣鼓这个python-patterns

设计模式对我来说更多的是学习而不是我的个人经验总结，所以我很可能理解偏，如果有大佬见到了请及时指出，我之所以选择在掘金来写一些个人的东西是因为这里的文章质量更高，我不希望后来者看到了这些文章被误导。

备忘录模式

from copy import copy, deepcopy


def memento(obj, deep=False):
    state = deepcopy(obj.__dict__) if deep else copy(obj.__dict__)

    def restore():
        obj.__dict__.clear()
        obj.__dict__.update(state)

    return restore


class Transaction:
    """A transaction guard.
    This is, in fact, just syntactic sugar around a memento closure.
    """

    deep = False
    states = []

    def __init__(self, deep, *targets):
        self.deep = deep
        self.targets = targets
        self.commit()

    def commit(self):
        self.states = [memento(target, self.deep) for target in self.targets]

    def rollback(self):
        for a_state in self.states:
            a_state()


class Transactional:
    """Adds transactional semantics to methods. Methods decorated  with
    @Transactional will rollback to entry-state upon exceptions.
    """

    def __init__(self, method):
        self.method = method

    def __get__(self, obj, T):
        def transaction(*args, **kwargs):
            state = memento(obj)
            try:
                return self.method(obj, *args, **kwargs)
            except Exception as e:
                state()
                raise e

        return transaction


class NumObj:
    def __init__(self, value):
        self.value = value

    def __repr__(self):
        return f"<{self.__class__.__name__}: {self.value!r}>"

    def increment(self):
        self.value += 1

    @Transactional
    def do_stuff(self):
        self.value = "1111"  # <- invalid value
        self.increment()  # <- will fail and rollback


def main():
    """
    >>> num_obj = NumObj(-1)
    >>> print(num_obj)
    <NumObj: -1>
    >>> a_transaction = Transaction(True, num_obj)
    >>> try:
    ...    for i in range(3):
    ...        num_obj.increment()
    ...        print(num_obj)
    ...    a_transaction.commit()
    ...    print('-- committed')
    ...    for i in range(3):
    ...        num_obj.increment()
    ...        print(num_obj)
    ...    num_obj.value += 'x'  # will fail
    ...    print(num_obj)
    ... except Exception:
    ...    a_transaction.rollback()
    ...    print('-- rolled back')
    <NumObj: 0>
    <NumObj: 1>
    <NumObj: 2>
    -- committed
    <NumObj: 3>
    <NumObj: 4>
    <NumObj: 5>
    -- rolled back
    >>> print(num_obj)
    <NumObj: 2>
    >>> print('-- now doing stuff ...')
    -- now doing stuff ...
    >>> try:
    ...    num_obj.do_stuff()
    ... except Exception:
    ...    print('-> doing stuff failed!')
    ...    import sys
    ...    import traceback
    ...    traceback.print_exc(file=sys.stdout)
    -> doing stuff failed!
    Traceback (most recent call last):
    ...
    TypeError: ...str...int...
    >>> print(num_obj)
    <NumObj: 2>
    """


if __name__ == "__main__":
    import doctest

    doctest.testmod(optionflags=doctest.ELLIPSIS)

观察者模式

from __future__ import annotations

from contextlib import suppress
from typing import List, Optional, Protocol


# define a generic observer type
class Observer(Protocol):
    def update(self, subject: Subject) -> None:
        pass


class Subject:
    def __init__(self) -> None:
        self._observers: List[Observer] = []

    def attach(self, observer: Observer) -> None:
        if observer not in self._observers:
            self._observers.append(observer)

    def detach(self, observer: Observer) -> None:
        with suppress(ValueError):
            self._observers.remove(observer)

    def notify(self, modifier: Optional[Observer] = None) -> None:
        for observer in self._observers:
            if modifier != observer:
                observer.update(self)


class Data(Subject):
    def __init__(self, name: str = "") -> None:
        super().__init__()
        self.name = name
        self._data = 0

    @property
    def data(self) -> int:
        return self._data

    @data.setter
    def data(self, value: int) -> None:
        self._data = value
        self.notify()


class HexViewer:
    def update(self, subject: Data) -> None:
        print(f"HexViewer: Subject {subject.name} has data 0x{subject.data:x}")


class DecimalViewer:
    def update(self, subject: Data) -> None:
        print(f"DecimalViewer: Subject {subject.name} has data {subject.data}")


def main():
    """
    >>> data1 = Data('Data 1')
    >>> data2 = Data('Data 2')
    >>> view1 = DecimalViewer()
    >>> view2 = HexViewer()
    >>> data1.attach(view1)
    >>> data1.attach(view2)
    >>> data2.attach(view2)
    >>> data2.attach(view1)
    >>> data1.data = 10
    DecimalViewer: Subject Data 1 has data 10
    HexViewer: Subject Data 1 has data 0xa
    >>> data2.data = 15
    HexViewer: Subject Data 2 has data 0xf
    DecimalViewer: Subject Data 2 has data 15
    >>> data1.data = 3
    DecimalViewer: Subject Data 1 has data 3
    HexViewer: Subject Data 1 has data 0x3
    >>> data2.data = 5
    HexViewer: Subject Data 2 has data 0x5
    DecimalViewer: Subject Data 2 has data 5
    # Detach HexViewer from data1 and data2
    >>> data1.detach(view2)
    >>> data2.detach(view2)
    >>> data1.data = 10
    DecimalViewer: Subject Data 1 has data 10
    >>> data2.data = 15
    DecimalViewer: Subject Data 2 has data 15
    """


if __name__ == "__main__":
    import doctest

    doctest.testmod()

小结

参考文献

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