def iter_tag_users(self, tag_id, first_user_id=None):
        """
        获取标签下粉丝openid列表

        :return: 返回一个迭代器，可以用for进行循环，得到openid

        使用示例::

            from wechatpy import WeChatClient

            client = WeChatClient('appid', 'secret')
            for openid in client.tag.iter_tag_users(0):
                print(openid)

        """
        while True:
            follower_data = self.get_tag_users(tag_id, first_user_id)
            if 'data' not in follower_data:
                return
            for openid in follower_data['data']['openid']:
                yield openid
            first_user_id = follower_data.get('next_openid')
            if not first_user_id:
                return