code: def id_to_extended_id(item_id, item_class):
          
          out = ID_PREFIX[item_class]
          if out:
              out += item_id
          return out
docs: Return the extended ID from an ID.
  
          :param item_id: The ID of the music library item
          :type item_id: str
          :param cls: The class of the music service item
          :type cls: Sub-class of
              :py:class:`soco.data_structures.MusicServiceItem`
  
          The extended id can be something like 00030020trackid_22757082
          where the id is just trackid_22757082. For classes where the prefix is
          not known returns None.