docs: def _missing_value_message(self, parent):
          # type: (ET.Element) -> Text
          """Return the message to report that the value needed for serialization is missing."""
          if self._attribute is None:
              message = 'Missing required value for element "{}"'.format(self.element_path)
          else:
              if self.element_path == '.':
                  parent_name = parent.tag
              else:
                  parent_name = self.element_path
  
              message = 'Missing required value for attribute "{}" on element "{}"'.format(
                  self._attribute, parent_name)
  
          return message