code: public PythonSingleOutputStreamOperator filter(FilterFunction<PyObject> filter) throws IOException {
  		return new PythonSingleOutputStreamOperator(stream.filter(new PythonFilterFunction(filter)));
  	}
docs: A thin wrapper layer over {@link DataStream#filter(FilterFunction)}.
  
  @param filter The FilterFunction that is called for each element of the DataStream.
  @return The filtered {@link PythonDataStream}.