返回的字符串为什么需要模上（title，...,publish_date）

return_str = 'title: %s, beief_content: %s,'\
             'content: %s, article_id: %s, publish_date: %s' % (title,
                                                           beief_content,
                                                           content,
                                                           article_id,
                                                           publish_date)