PythonMonkey: Seamless Interoperability Between Python, JavaScript, and WebAssembly

PythonMonkey is a Python library that leverages Mozilla’s SpiderMonkey JavaScript engine to provide high‑performance, bidirectional interoperability between Python, JavaScript, and WebAssembly. It allows JavaScript code to execute Python snippets and vice‑versa with almost no measurable performance loss.

The library includes PMJS , a Node.js‑like runtime for JavaScript that can import and call Python modules, and supports executing WebAssembly modules directly from Python. This makes it possible to call popular JavaScript packages such as crypto‑js from Python, or use Python libraries like NumPy inside JavaScript.

Example "hello world" in PythonMonkey:

import pythonmonkey as pm
hello = pm.eval("'Hello World'.toUpperCase();")
print(hello)  # 'HELLO WORLD'

A more complex example shows passing Python's print function to a JavaScript callback:

import pythonmonkey as pm
hello = pm.eval("(func) => { func('Hello World!') }")
hello(print)  # prints Hello World!

Developers can also expose JavaScript modules to Python. For instance, a simple JS module my‑javascript‑module.js :

exports.sayHello = () => { console.log('hello, world'); };

and use it from Python:

import pythonmonkey as pm
test = pm.require('./my-javascript-module')
test.sayHello()  # prints hello, world

Conversely, Python modules can be required from JavaScript using CommonJS:

# my‑python‑module.py
def getStringLength(s):
    return len(s)
exports['getStringLength'] = getStringLength

// my‑javascript‑module.js
const { getStringLength } = require('./my-python-module');
function printStringLength(s) {
    console.log(`String "${s}" has a length of ${getStringLength(s)}`);
}
module.exports = { printStringLength };

import pythonmonkey as pm
test = pm.require('./my-javascript-module')
test.printStringLength('Hello, world!')  # String "Hello, world!" has a length of 13

PythonMonkey also integrates WebAssembly APIs, allowing asynchronous loading and execution of WASM modules from Python, as demonstrated with a factorial example.

Compared with alternative projects such as JS2Py, PyV8, and Metacall, PythonMonkey offers superior speed (over 1,162× faster than JS2Py in SunSpider benchmarks), full support for modern JavaScript features like Promises and async/await, and efficient data passing via references rather than copying.

References: PythonMonkey GitHub repository and example collection .