code: def normalize_val(self, value=_nothing):
          
          if isinstance(value, basestring):
              value = self.normalize_text(value)
          if self.ignore_empty_slots and self.value_is_empty(value):
              value = _nothing
          return value
docs: 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.