def children(self):
        """获取此话题的子话题

        :return: 此话题的子话题， 返回生成器
        :rtype: Topic.Iterable
        """
        self._make_soup()
        child_topic_tag = self.soup.find('div', class_='child-topic')
        if child_topic_tag is None:
            return []
        elif '共有' not in child_topic_tag.contents[-2].text:
            for topic_tag in child_topic_tag.div.find_all('a'):
                yield Topic(Zhihu_URL + topic_tag['href'],
                            topic_tag.text.strip(),
                            session=self._session)
        else:
            flag = 'load'
            child = ''
            data = {'_xsrf': self.xsrf}
            params = {
                'parent': self.id
            }
            while flag == 'load':
                params['child'] = child
                res = self._session.post(Topic_Get_Children_Url,
                                         params=params, data=data)
                j = map(lambda x: x[0], res.json()['msg'][1])
                *topics, last = j
                for topic in topics:
                    yield Topic(Zhihu_URL + '/topic/' + topic[2], topic[1],
                                session=self._session)
                flag = last[0]
                child = last[2]
                if flag == 'topic':
                    yield Topic(Zhihu_URL + '/topic/' + last[2], last[1],
                                session=self._session)
获取此话题的子话题

        :return: 此话题的子话题， 返回生成器
        :rtype: Topic.Iterable