code: def unanswered_questions(self):
          
          from .question import Question
          question_url = Topic_Unanswered_Question_Url.format(self.id)
          params = {'page': 1}
          while True:
              res = self._session.get(question_url, params=params)
              soup = BeautifulSoup(res.content)
              if soup.find('div', class_='error') is not None:
                  return
              questions = soup.find_all('div', class_='question-item')
              for qu_div in questions:
                  url = Zhihu_URL + qu_div.h2.a['href']
                  title = qu_div.h2.a.text.strip()
                  yield Question(url, title, session=self._session)
              params['page'] += 1
docs: 获取话题下的等待回答的问题
  
          什么是「等待回答」的问题：https://www.zhihu.com/question/40470324
  
          :return: 话题下等待回答的问题，返回生成器
          :rtype: Question.Iterable