code: def get_tracks(self):
          
          if not self._track_list:
              tracks = itunespy.lookup(id=self.collection_id, entity=itunespy.entities['song'])[1:]
              for track in tracks:
                  self._track_list.append(track)
          return self._track_list
docs: Retrieves all the tracks of the album if they haven't been retrieved yet
          :return: List. Tracks of the current album