code: def _comparison(self, op, value):
      
      # NOTE: This is also used by query.gql().
      if not self._indexed:
        raise datastore_errors.BadFilterError(
            'Cannot query for unindexed property %s' % self._name)
      from .query import FilterNode  # Import late to avoid circular imports.
      if value is not None:
        value = self._do_validate(value)
        value = self._call_to_base_type(value)
        value = self._datastore_type(value)
      return FilterNode(self._name, op, value)
docs: Internal helper for comparison operators.
  
      Args:
        op: The operator ('=', '<' etc.).
  
      Returns:
        A FilterNode instance representing the requested comparison.