code: def set_dimmer(self, dimmer, transition_time=None):
          
          values = {
              ATTR_LIGHT_DIMMER: dimmer,
          }
          if transition_time is not None:
              values[ATTR_TRANSITION_TIME] = transition_time
          return self.set_values(values)
docs: Set dimmer value of a group.
  
          dimmer: Integer between 0..255
          transition_time: Integer representing tenth of a second (default None)