def reply_topic(self, topic_id, content):
        """小组回帖

        :return: 帖子 id 或 ``None``
        """
        data = {
            'body': content,
            'csrfmiddlewaretoken': self._request.cookies.get('csrftoken')
        }
        url = 'http://www.shanbay.com/api/v1/forum/thread/%s/post/' % topic_id
        r = self.request(url, 'post', data=data)
        j = r.json()
        if j['status_code'] == 0:
            return j['data']['thread']['id']