memento.py

"""code.activestate.com/recipes/413838-memento-closure/"""

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:
    """A transaction guard. This is really just
      syntactic suggar arount a memento closure.
      """
    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):
    """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:
                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 DoStuff(self):
        self.value = '1111'  # <- invalid value
        self.Increment()     # <- will fail and rollback


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.DoStuff()
    except:
        print('-> doing stuff failed!')
        import traceback
        traceback.print_exc(0)
        pass
    print(n)