code: def get_version(self, app_id, version):
          
          response = self._do_request('GET', '/v2/apps/{app_id}/versions/{version}'
                                      .format(app_id=app_id, version=version))
          return MarathonApp.from_json(response.json())
docs: Get the configuration of an app at a specific version.
  
          :param str app_id: application ID
          :param str version: application version
  
          :return: application configuration
          :rtype: :class:`marathon.models.app.MarathonApp`