docs: def _add_subscribers_for_type(self, callback_type, subscribers, callbacks, **kwargs):
          ''' add a done/queued/progress callback to the appropriate list '''
          for subscriber in subscribers:
              callback_name = 'on_' + callback_type
              if hasattr(subscriber, callback_name):
                  _function = functools.partial(getattr(subscriber, callback_name), **kwargs)
                  callbacks.append(_function)