code: public PythonKeyedStream key_by(KeySelector<PyObject, PyKey> selector) throws IOException {
  		return new PythonKeyedStream(stream.keyBy(new PythonKeySelector(selector)));
  	}
docs: A thin wrapper layer over {@link DataStream#keyBy(KeySelector)}.
  
  @param selector The KeySelector to be used for extracting the key for partitioning
  @return The {@link PythonDataStream} with partitioned state (i.e. {@link PythonKeyedStream})