code: def top_answers(self):
          
          from .question import Question
          from .answer import Answer
          from .author import Author, ANONYMOUS
  
          top_answers_url = Topic_Top_Answers_Url.format(self.id)
          params = {'page': 1}
          while True:
              # 超出50页直接返回
              if params['page'] > 50:
                  return
              res = self._session.get(top_answers_url, params=params)
              params['page'] += 1
              soup = BeautifulSoup(res.content)
              # 不够50页，来到错误页面 返回
              if soup.find('div', class_='error') is not None:
                  return
              questions = soup.find_all('a', class_='question_link')
              answers = soup.find_all('a', class_='answer-date-link')
              authors = soup.find_all('div', class_='zm-item-answer-author-info')
              upvotes = soup.find_all('a', class_='zm-item-vote-count')
              for ans, up, q, au in zip(answers, upvotes, questions, authors):
                  answer_url = Zhihu_URL + ans['href']
                  question_url = Zhihu_URL + q['href']
                  question_title = q.text.strip()
                  upvote = up.text
                  if upvote.isdigit():
                      upvote = int(upvote)
                  else:
                      upvote = None
                  question = Question(question_url, question_title,
                                      session=self._session)
                  if au.a is None:
                      author = ANONYMOUS
                  else:
                      author_url = Zhihu_URL + au.a['href']
                      author_name = au.a.text
                      author_motto = au.strong['title'] if au.strong else ''
                      author = Author(author_url, author_name, author_motto,
                                      session=self._session)
                  yield Answer(answer_url, question, author, upvote,
                               session=self._session)
docs: 获取话题下的精华答案.
  
          :return: 话题下的精华答案，返回生成器.
          :rtype: Answer.Iterable