docs: def __get_isbn(self, html):
          """
          从图书借阅状态页面中获取isbn
          :param html:
          :return:
          """
  
          import re
  
          reg = re.compile(r'getBookCover\(".*","(.*)"\);')
          res = reg.findall(html)
          if len(res) > 0:
              return res[0]
          else:
              return ''