Jython support in JBang

[wfouche]

Is anyone interested in seeing Jbang support Jython?

• https://www.jython.org/
• https://repo1.maven.org/maven2/org/python/jython-standalone/2.7.4/

An extension for Python to use JBang already exists, but I'm proposing the inverse of the functionality provided by that project.

• https://www.jbang.dev/learn/python-with-jbang/

Recently I wrote a Jython (Python) script to process a JSON file that contains amongst other data structures several persistent images of HdrHistograms. The output generated by the report.py program is an HTML report.

The report.py file is executed with Java and Jython as follows:

$ java -Dpython.path=./jars/HdrHistogram-2.2.2.jar -jar ./jars/jython-standalone-2.7.4.jar  report.py

If JBang supported Jython, the Jbang augmented report.py file would look something like this:

///usr/bin/env jbang "$0" "$@" ; exit $?
//DEPS org.hdrhistogram:HdrHistogram:2.2.2
//JAVA 21
//JYTHON 2.7.4

from collections import OrderedDict
import datetime
import json
import sys
import org.HdrHistogram.Histogram as Histogram

def createReport(filename):
   jh = Histogram(1, 3600*1000*1000, 3)
   fh = open(filename)
   jb = json.load(fh, object_pairs_hook=OrderedDict)
   # ....
   # ....

if __name__ == "__main__":
    filename = sys.argv[1]
    createReport(filename)

The report.py file can then be executed as:

$ jbang run report.py

[maxandersen]

problem with using jython is that jython only supports python 2 afaik ?

are you aware of the graalpy integration? See https://github.com/oracle/graalpython/blob/master/graalpython/graalpy-jbang/examples/hello.java

I've always wondered if something as you suggest above would be feasible via graalpy as graalpy does support later more recent python versions.

[wfouche]

I've always wondered if something as you suggest above would be feasible via
graalpy as graalpy does support later more recent python versions.

@maxandersen , I'll experiment with graalpy in the context of jbang. Will share the results if it looks promising. My immediate requirements are met by Jython, and getting it to run effortlessly on Windows, Linux and MacOS with jbang.

[maxandersen]

cool - either way its good stuff to get discussed to see what we can do.

[wfouche]

Yes, Jython 2.7.4 is the latest stable release, but it contains at least some Python 3 functionality like the print() function and so forth. But it is not Python 3.x

A few years ago, I did look at graalpython as a potential Jython replacement, but it still had many rough edges. It should have improved a lot by now. Worth having another look.

It seems that adding Jython support would be relatively simple, and very similar to how Jshell is supported.

[wfouche]

Documentation discussing support for Jython in the GraalVM - https://www.graalvm.org/jdk17/reference-manual/python/Jython/

[maxandersen]

So would a graalpy .py variant work ?

[wfouche]

so would a graalpy .py variant work ?

It is a bit cumbersome to use graalpy (for me), but I have found a workaround solution using jpm (https://github.com/codejive/java-jpm) which incidently can be installed via jbang.

[quintesse]

Understood. So you need this extra step because Jython doesn't allow you to do -Dpython.path=deps/*? Because java allows the usage of wildcards, so you can simply do -cp deps/* (it's not the shell handling this, so it also works on Windows)

[quintesse]

Btw, jpm path will print out the path for you, maybe that makes it a bit easier?

Edit: running this without first running jpm install will print out the paths directly to the Maven artifacts in your ~/.m2 folder. In case you're not really interested in having the artifacts copied locally in ./deps/

[wfouche]

So you need this extra step because Jython doesn't allow you to do -Dpython.path=deps/*?

Correct. But -cp deps/* does not work because Jython only looks at JARs that are in python.path.

Btw, jpm path will print out the path for you, maybe that makes it a bit easier?

Thanks. It does. I tried the following and it worked.

java -Dpython.path=`jbang run jpm@codejive path` -jar  ....

[wfouche]

A simpler and pure JBang solution is possible using my (by now) better understanding of the Jython source code. I had a look at the Jython Java source code, and discovered a simply way of bootstrapping Jython via a Jbang Java script:

Jython.java

///usr/bin/env jbang "$0" "$@" ; exit $?

//DEPS com.google.code.gson:gson:2.11.0
//DEPS org.hdrhistogram:HdrHistogram:2.2.2

//DEPS org.python:jython-standalone:2.7.4

import org.python.util.jython;

public class Jython {
    public static void main(String... args) {
        jython.main(args);
    }
}

Any jython script can then be run with command:

$ jbang run Jython.java <script.py> <param1> <param2> ...

[quintesse]

Great!

Btw, that also explains why this didn't work:

Correct. But -cp deps/* does not work because Jython only looks at JARs that are in python.path.

It's because you can't combine -cp with -jar, you'd have to rewrite it like this (assuming jython-standalone-2.7.4.jar can be found in deps as well):

$ java  \
  -cp=deps/* \
  org.python.util.jython \
  report2.py \
  report2_config.jsonc

[wfouche]

python-jvm@wfouche is a JBang app that streamlines the process of executing Jython programs within a JVM environment.

#!/usr/bin/env python-jvm

# restclient_jython.py
#
# Run: jbang run python-jvm@wfouche restclient_jython.py
#

# /// jbang
# requires-jython = "==2.7.4"
# requires-java = ">=21"
# debug = false
# dependencies = [
#   "org.springframework.boot:spring-boot-starter-web:3.4.4"
# ]
# [java]
#   runtime-options = "-server -Xms2g -Xmx2g -XX:+UseZGC -XX:+ZGenerational"
# ///

import java.lang.String as String
import org.springframework.web.client.RestClient as RestClient

def restApiCall(uri, id):
    restClient = RestClient.create()
    rsp = restClient.get().uri(uri, id).retrieve().body(String)
    print(rsp)

def main():
    restApiCall("https://jsonplaceholder.typicode.com/todos/{id}", 1)

main()

Output:

{
  "userId": 1,
  "id": 1,
  "title": "delectus aut autem",
  "completed": false
}

[wfouche]

Running python-jvm without any parameters will start the Jython interpreter.

$ jbang run python-jvm@wfouche
python-jvm/1/2025-04-24T17:58:24
Jython 2.7.4 (tags/v2.7.4:3f256f4a7, Aug 18 2024, 10:30:53)
[OpenJDK 64-Bit Server VM (Eclipse Adoptium)] on java21.0.6
Type "help", "copyright", "credits" or "license" for more information.
>>> 
>>> print(1+1)
2
>>> 
>>>