def _comparison(self, op, value):
<EOL>if not self._indexed:<EOL><INDENT>raise datastore_errors.BadFilterError(<EOL>'<STR_LIT>' % self._name)<EOL><DEDENT>from .query import FilterNode  <EOL>if value is not None:<EOL><INDENT>value = self._do_validate(value)<EOL>value = self._call_to_base_type(value)<EOL>value = self._datastore_type(value)<EOL><DEDENT>return FilterNode(self._name, op, value)<EOL>
Internal helper for comparison operators.

        Args:
          op: The operator ('=', '<' etc.).

        Returns:
          A FilterNode instance representing the requested comparison.