code: def trackers(self):
          
          announce_list = self.metainfo.get('announce-list', None)
          if not announce_list:
              announce = self.metainfo.get('announce', None)
              if announce:
                  return [[announce]]
          else:
              return announce_list
docs: List of tiers of announce URLs or ``None`` for no trackers
  
          A tier is either a single announce URL (:class:`str`) or an
          :class:`~collections.abc.Iterable` (e.g. :class:`list`) of announce
          URLs.
  
          Setting this property sets or removes ``announce`` and ``announce-list``
          in :attr:`metainfo`. ``announce`` is set to the first tracker of the
          first tier.
  
          :raises URLError: if any of the announce URLs is invalid