def normalize_val(self, value=_nothing):
        """Hook which is called on every value before comparison, and should
        return the scrubbed value or ``self._nothing`` to indicate that the
        value is not set.
        """
        if isinstance(value, basestring):
            value = self.normalize_text(value)
        if self.ignore_empty_slots and self.value_is_empty(value):
            value = _nothing
        return value