code: def _check_field_names(self, field_names):
          
          if field_names:
              for field_name in field_names:
                  try:
                      self.column[field_name]
                  except KeyError:
                      raise InvalidIndexError('Trying to use non indexed field "%s"' % field_name)
docs: Raises InvalidIndexError if any of a field_name in field_names is
          not indexed.