docs: def _get_assessment_part(self, part_id=None):
          """Gets an AssessmentPart given a part_id.
  
          Returns this Section's own part if part_id is None.
  
          Make this a private part, so that it doesn't collide with the AssessmentPart.get_assessment_part
          method, which does not expect any arguments...
  
          """
          if part_id is None:
              return self._assessment_part
          if part_id not in self._assessment_parts:
              lookup_session = self._get_assessment_part_lookup_session()
              self._assessment_parts[part_id] = lookup_session.get_assessment_part(part_id)
          return self._assessment_parts[part_id]