code: def update_command(self, command, args=None):
          
          if command is None:
              self._command = lambda: None
          else:
              if args is None:
                  self._command = command
              else:
                  self._command = utils.with_args(command, *args)
docs: Updates the callback command which is called when the ButtonGroup
          changes.
  
          Setting to `None` stops the callback.
  
          :param callback command:
              The callback function to call.
  
          :param callback args:
              A list of arguments to pass to the widgets `command`, defaults to
              `None`.