pyjs

pyjs is a modern pybind11 + emscripten embind based Python <-> JavaScript foreign function interface (FFI) for wasm/emscripten compiled Python. The API is loosely based on the FFI of pyodide.

• Python 3.14, emscripten 6, wasm32 and wasm64
• runs in the browser main thread and in web workers
• asyncio support: await JavaScript promises from Python and Python coroutines from JavaScript
• tested in Chromium, Firefox and WebKit (Safari)
• distributed via emscripten-forge

Documentation

See the documentation.

Try it out

• the Python REPL in the docs
• the JupyterLite playground
• to create your own JupyterLite deployments, use xeus-lite-demo

Installation

pyjs is installed into an emscripten-forge environment:

# environment.yml
name: my-pyjs-env
channels:
  - https://repo.prefix.dev/emscripten-forge/6x
  - conda-forge
dependencies:
  - pyjs-dev
  - numpy

micromamba create -f environment.yml --platform emscripten-wasm32 --prefix /path/to/env

See deploying pyjs for how to pack the environment with empack and load it in the browser.

Quickstart

Access JavaScript from Python:

import pyjs

# hello world
pyjs.js.console.log("Hello, World!")

# create a JavaScript function to add two numbers
js_function = pyjs.js.Function("a", "b", """
    console.log("Adding", a, "and", b)
    return a + b
""")

# call the function
result = js_function(1, 2)

Access Python from JavaScript:

// hello world
pyjs.eval("print('Hello, World!')")

// eval a python expression and get the result
const py_list = pyjs.eval("[i for i in range(10)]")

// access
console.log(py_list.get(0)) // same as py_list[0] on the python side

Development

Building, testing, benchmarking and building the docs all go through build_local.sh (only micromamba is needed), the CI uses the same script:

./build_local.sh wasm32                 # build pyjs
./build_local.sh wasm64 --test          # build + run the tests in the browser
BROWSERS="chromium firefox webkit" ./build_local.sh wasm32 --test
./build_local.sh wasm32 --bench         # FFI / asyncio benchmarks
./build_local.sh wasm32 --docs          # sphinx docs + jupyterlite

See development for details.