docs: def add_subscribers(self, subscribers, **kwargs):
          """ Add a callbacks to be invoked during transfer """
          if subscribers:
              with self._callbacks_lock:
                  self._add_subscribers_for_type(
                      'done', subscribers, self._done_callbacks, **kwargs)
                  self._add_subscribers_for_type(
                      'queued', subscribers, self._queued_callbacks, **kwargs)
                  self._add_subscribers_for_type(
                      'progress', subscribers, self._progress_callbacks, **kwargs)