Compile Python to WebAssembly (WASM)

Python documentation: Tools/wasm/README.md.

April 2022: There is a work-in-progress to support WASM targets in Python 3.11 which is still at the alpha stage (release scheduled in October 2022).

Targets

Status in April 2022 (from Christian Heimes’ slides at Pycon DE):

Features

CPython Emscripten browser

CPython Emscripten node

Pyodide

subprocess (fork, exec)

no

no

no

threads

no

YES

WIP

file system

no (only MEMFS)

YES (Node raw FS)

YES (IDB, Node, …)

shared extension modules

WIP

WIP

YES

PyPI packages

no

no

YES

sockets

?

?

?

urllib, asyncio

no

no

WebAPI fetch / WebSocket

signals

no

WIP

YES

CPython on WebAssembly

Status

  • WARNING: WASM support is highly experimental! Lots of features are not working yet.

  • April 2022: Christian Heime’s talk at Pycon DE: Python 3.11 in the web browser - A journey

  • April 2022: WASM binary is ~4.5 MB compressed with static, reduced stdlib. 4.6 MB wasm (compressed 1.5 MB), 2.7 MB stdlib bytecode, some JS.

Python on wasm32-emscripten

  • sys.platform is 'emscripten'

  • os.name is 'posix'

  • platform.machine() is 'wasm32'

  • Example of platform.platform(): 'Emscripten-1.0-wasm32-32bit'

  • Example os.uname(): posix.uname_result(sysname='Emscripten', nodename='emscripten', release='1.0', version='#1', machine='wasm32')

Python on wasm32-wasi

  • sys.platform is 'wasi'

  • os.name is 'posix'

  • platform.machine() is 'wasm32'

  • Example of platform.platform(): 'wasi-0.0.0-wasm32-32bit'

  • Example os.uname(): posix.uname_result(sysname='wasi', nodename='(none)', release='0.0.0', version='0.0.0', machine='wasm32')

Build

Cross-build CPython to WASM:

In short, on Fedora, commands to type in the Python source code directory:

git clean -fdx  # warning: it removes ALL untracked files!
podman run --rm -ti -v $(pwd):/python-wasm/cpython:Z -w /python-wasm/cpython quay.io/tiran/cpythonbuild:emsdk3

In the emsdk3 container:

# python3 -u ./Tools/wasm/wasm_build.py wasi build -v 2>&1|tee log
./Tools/wasm/wasm_build.py wasi build # -v
cd builddir/wasi/
make pythoninfo
make buildbottest TESTOPTS=""

Build 2

Documentation: https://devguide.python.org/getting-started/setup-building/#wasi

Create a container from “devcontainer” cpython-dev (container based on Fedora):

git clone https://github.com/python/cpython-devcontainers
cd cpython-devcontainers
podman build devcontainer --tag wasi-cpython-dev

In a CPython checkout, start by removing all local files not tracked by Git:

git clean -fdx

In the clean CPython checkout, run the container:

podman run -it --rm -v $PWD:/workspace:O -w/workspace wasi-cpython-dev

Now inside the container, build Python for WASI:

python3 Tools/wasm/wasi.py build -- --config-cache --with-pydebug

Run Python:

cross-build/wasm32-wasip1/python.sh

Run the test suite:

cross-build/wasm32-wasip1/python.sh -m test

Python WASM browser REPL

Christian Heimes’ REPL:

Ethan Smith’s REPL

Python WASM detailed status

Disabled C extensions

C extensions disabled on Emscripten/browser, Emscripten/node and WASI targets:

  • _ctypes, _ctypes_test

  • _curses, _curses_panel

  • _dbm, _gdbm

  • _scproxy

  • _tkinter

  • _xxsubinterpreters

  • grp

  • nis

  • ossaudiodev

  • pwd, spwd

  • syslog

C extensions disabled on Emscripten/browser, Emscripten/node targets:

  • _multiprocessing

  • _posixshmem

  • _posixsubprocess

C extensions disabled on the Emscripten/browser target:

  • fcntl

  • readline

  • resource

  • termios

Pyodide

Python distribution for the browser and Node.js based on WebAssembly:

WASI

pygame and WASM

Misc