Python class attribute setter

Question

class Foo(object):

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

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

what I'd like to do is:

1. when read from self.bar[xxx], I get the real value

2. when write to self.bar[xxx], call a setter to do the real setting.

Here is a real world example:

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

the problem is, I don't want to check if self._vote_nums['up or down'] is None each time I want to write to it.