CPython on WASM

Build scripts and configuration for building CPython for Emscripten.

Check out Christian Heimes' talk about the effort at PyConDE: https://www.youtube.com/watch?v=oa2LllRZUlU

Pretty straight forward. First, install emscripten. Then, run the following commands:

# get the Python sources
./fetch-python.sh
# build Python for the machine we are building on, needed before cross compiling for emscripten
./build-python-build.sh
# build Python cross-compiling to emscripten
./build-python-emscripten-browser.sh

There will probably be errors, but that's just part of the fun of experimental platforms.

Assuming things compiled correctly, you can have emscripten serve the Python executable and then open http://localhost:8000/python.html in your browser:

./run-python-browser.sh

The CLI input is done via an input modal which is rather annoying. Also to get output you need to click Cancel on the modal...

Developing

Once you've built the Emscripten'd Python, you can rebuild it via

./clean-host.sh
./build-python-emscripten-browser.sh

which will rebuild Python targeting emscripten and re-generate the python.{html, wasm, js}

Test build artifacts

You can also download builds from our CI workflow and test WASM builds locally.

Emscripten browser build

• download and unzip the emscripten-browser-main.zip build artifact
• run a local webserver in the same directory as python.html, e.g. python3 -m http.server
• open http://localhost:8000/python.html
• enter commands into the browser modal window and check the web developer console (F12) for output. You may need to hit "Cancel" on the modal after sending input for output to appear.

Emscripten NodeJS build

• download and unzip the emscripten-node-main.zip build artifact
• run node python.js (older versions may need --experimental-wasm-bigint)

WASI

• download and unzip the wasi-main.zip build artifact
• install wasmtime
• run wasmtime run --dir . -- python.wasm