code: @Override
  	public final void coGroup(Iterable<IN1> first, Iterable<IN2> second, Collector<OUT> out) throws Exception {
  		streamer.streamBufferWithGroups(first.iterator(), second.iterator(), out);
  	}
docs: Calls the external python function.
  
  @param first
  @param second
  @param out collector
  @throws IOException