code: def _get_json_field(self, response_json, field_name):
          
          if field_name not in response_json:
              raise KeyError('Unable to get value for "%s" from Marathon '
                             'response: "%s"' % (
                                 field_name, json.dumps(response_json),))
  
          return response_json[field_name]
docs: Get a JSON field from the response JSON.
  
          :param: response_json:
              The parsed JSON content of the response.
          :param: field_name:
              The name of the field in the JSON to get.