code: def announce_urls(self):
          
          urls = self._struct.get('announce-list')
  
          if not urls:
              urls = self._struct.get('announce')
              if not urls:
                  return []
              urls = [[urls]]
  
          return urls
docs: List of lists of announce (tracker) URLs.
  
          First inner list is considered as primary announcers list,
          the following lists as back-ups.
  
          http://bittorrent.org/beps/bep_0012.html