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:

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