python设计模式之备忘录

python设计模式之备忘录

意图

• 在不破坏封装性的前提下，捕获一个对象的内部状态，并在该对象之外保存这个状态。这样以后就可将该对象恢复到原先保存的状态

适用性

• 必须保存一个对象在某一个时刻的(部分)状态, 这样以后需要时它才能恢复到先前的状态，如果一个用接口来让其它对象直接得到这些状态，将会暴露对象的实现细节并破坏对象的封装性。

例子

import copy


def Memento(obj, deep=False):
    state = (copy.copy, copy.deepcopy)[bool(deep)](obj.__dict__)

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

    return restore


class Transaction:
    deep = False

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

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

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


class transactional(object):
    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:
                state()
                raise

        return transaction


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

    def __repr__(self):
        return '<%s: %r>' % (self.__class__.__name__, self.value)

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

    @transactional
    def do_stuff(self):
        self.value = '1111'
        self.increment()


if __name__ == '__main__':
    n = NumObj(-1)
    print(n)
    t = Transaction(n)
    try:
        for i in range(3):
            n.increment()
            print(n)
        t.commit()
        print('-- commited')
        for i in range(3):
            n.increment()
            print(n)
        n.value += 'x'  # will fail
        print(n)
    except:
        t.rollback()
        print('-- rolled back')
    print(n)


    print('-- now doing stuff ...')
    try:
        n.do_stuff()
    except:
        print('-> doing stuff failed!')
        import traceback

        traceback.print_exc(0)
        pass
    print(n)

# output:
<NumObj: -1>
<NumObj: 0>
<NumObj: 1>
<NumObj: 2>
-- commited
<NumObj: 3>
<NumObj: 4>
<NumObj: 5>
-- rolled back
<NumObj: 2>
-- now doing stuff ...
-> doing stuff failed!
<NumObj: 2>
Traceback (most recent call last):
TypeError: cannot concatenate 'str' and 'int' objects