How to use the heronpy.api.stream.Grouping.custom function in heronpy

def _calculate_inputs(self):
    return {GlobalStreamId(self._parent.get_name(), self._parent._output) :
            Grouping.custom(RepartitionCustomGrouping(self._repartition_function))}

def _calculate_inputs(self):
    return {GlobalStreamId(self._parent.get_name(), self._parent._output) :
            Grouping.custom("heronpy.streamlet.impl.reducebywindowbolt.ReduceGrouping")}

def _calculate_inputs(self):
    return {GlobalStreamId(self._parent.get_name(), self._parent._output) :
            Grouping.custom("heronpy.streamlet.impl.reducebykeyandwindowbolt.ReduceGrouping")}

def prepare(self, context, component, stream, target_tasks):
    logging.getLogger().info("In prepare of SampleCustomGrouping, "
                             "with src component: %s, "
                             "with stream id: %s, "
                             "with target tasks: %s"
                             , component, stream, str(target_tasks))
    self.target_tasks = target_tasks

  def choose_tasks(self, values):
    # only emits to the first task id
    return [self.target_tasks[0]]

class CustomGrouping(Topology):
  word_spout = WordSpout.spec(par=1)
  consume_bolt = ConsumeBolt.spec(par=3,
                                  inputs={word_spout: Grouping.custom(SampleCustomGrouping())},
                                  config={constants.TOPOLOGY_TICK_TUPLE_FREQ_SECS: 10})