Python类属性setter

Question

class Foo(object):

    bar = {'a': None, 'b': None}

    def do_something(self):
        print self.bar['a']
        self.bar['a'] = 12

我想做的是：

1. 从self.bar[xxx]读取时，我得到了真正的价值

2. 写入self.bar[xxx]时，请致电制定者进行真实设置。

这是一个真实世界的例子：

class VoteMixin(object):

    self._vote_nums = {'up': None, 'down': None}

    def fetch_from_db(self):
        self._vote_nums = {} # set with values read from db

    @property
    def up_vote_nums(self):
        if self._vote_nums['up'] is None:
            self.fetch_from_db()
        return self._vote_nums['up']

    @property
    def down_vote_nums(self):
        # the same
        pass

    def up_vote(self):
        # update to db

        if self.vote_nums['up'] is None:
            self.fetch_from_db()

        self._vote_nums['up'] += 1

    def down_vote(self):
        # update to db

        if self.vote_nums['down'] is None:
            self.fetch_from_db()

        self._vote_nums['down'] += 1

问题是，我不想在每次写入时都检查self._vote_nums['up or down']是否为无。