code: def _evaluate(self):
          
          if self._elements:
              for element in self._elements:
                  yield element
          else:
              for user_id in self.__user_ids:
                  element = self._swimlane.users.get(id=user_id)
                  self._elements.append(element)
                  yield element
docs: Lazily retrieve and build User instances from returned data