code: def _propagate_index(self, change):
          "Propagate changes in index to the value and label properties"
          label = self._options_labels[change.new] if change.new is not None else None
          value = self._options_values[change.new] if change.new is not None else None
          if self.label is not label:
              self.label = label
          if self.value is not value:
              self.value = valuf _propagate_index(self, change):
          "Propagate changes in index to the value and label properties"
          label = self._options_labels[change.new] if change.new is not None else None
          value = self._options_values[change.new] if change.new is not None else None
          if self.label is not label:
              self.label = label
          if self.value is not value:
              self.value = value
docs: Propagate changes in index to the value and label properties