code: public PythonSingleOutputStreamOperator reduce(ReduceFunction<PyObject> reducer) throws IOException {
  		return new PythonSingleOutputStreamOperator(this.stream.reduce(new PythonReduceFunction(reducer)));
  	}
docs: A thin wrapper layer over {@link KeyedStream#reduce(ReduceFunction)}.
  
  @param reducer The {@link ReduceFunction} that will be called for every
  element of the input values with the same key.
  @return The transformed data stream @{link PythonSingleOutputStreamOperator}.