def refresh(self):
        """刷新 Question object 的属性. 
        例如回答数增加了, 先调用 ``refresh()`` 
        再访问 answer_num 属性, 可获得更新后的答案数量.
        
        :return: None
        """
        super().refresh()
        self._html = None
        self._title = None
        self._details = None
        self._answer_num = None
        self._follower_num = None
        self._topics = None
        self._last_edit_time = None
        self._logs = None