code: def topics(self):
          
          from .topic import Topic
  
          for topic in self.soup.find_all('a', class_='zm-item-tag'):
              yield Topic(Zhihu_URL + topic['href'], topic.text.replace('\n', ''),
                          session=self._session)
docs: 获取问题所属话题.
  
          :return: 问题所属话题
          :rtype: Topic.Iterable