code: def update_widget(self, idx=None):
          
          if idx is None:
              for w in self._widgets:
                  idx = self._get_idx_from_widget(w)
                  self._write_widget(self._read_property(idx), idx)
              pass
          else: self._write_widget(self._read_property(idx), idx)
          return
docs: Forces the widget at given index to be updated from the
          property value. If index is not given, all controlled
          widgets will be updated. This method should be called
          directly by the user when the property is not observable, or
          in very unusual conditions.