Python WebAssembly (WASM) build

WASI support is tier 2. Emscripten support is tier 3.

This directory contains configuration and helpers to facilitate cross compilation of CPython to WebAssembly (WASM). Python supports Emscripten (wasm32-emscripten) and WASI (wasm32-wasi) targets. Emscripten builds run in modern browsers and JavaScript runtimes like Node.js. WASI builds use WASM runtimes such as wasmtime.

Users and developers are encouraged to use the script Tools/wasm/wasm_build.py. The tool automates the build process and provides assistance with installation of SDKs, running tests, etc.

NOTE: If you are looking for information that is not directly related to building CPython for WebAssembly (or the resulting build), please see https://github.com/psf/webassembly for more information.

wasm32-emscripten

Build

To cross compile to the wasm32-emscripten platform you need the Emscripten compiler toolchain, a Python interpreter, and an installation of Node version 18 or newer. Emscripten version 4.0.2 is recommended; newer versions may also work, but all official testing is performed with that version. All commands below are relative to a checkout of the Python repository.

Install the Emscripten compiler toolchain

You can install the Emscripten toolchain as follows:

git clone https://github.com/emscripten-core/emsdk.git --depth 1
./emsdk/emsdk install latest
./emsdk/emsdk activate latest

To add the Emscripten compiler to your path:

source ./emsdk/emsdk_env.sh

This adds emcc and emconfigure to your path.

Optionally: enable ccache for EMSDK

The EM_COMPILER_WRAPPER must be set after the EMSDK environment is sourced. Otherwise the source script removes the environment variable.

export EM_COMPILER_WRAPPER=ccache

Compile and build Python interpreter

You can use python Tools/wasm/emscripten to compile and build targeting Emscripten. You can do everything at once with:

python Tools/wasm/emscripten build

or you can break it out into four separate steps:

python Tools/wasm/emscripten configure-build-python
python Tools/wasm/emscripten make-build-python
python Tools/wasm/emscripten make-libffi
python Tools/wasm/emscripten configure-host
python Tools/wasm/emscripten make-host

Extra arguments to the configure steps are passed along to configure. For instance, to do a debug build, you can use:

python Tools/wasm/emscripten build --with-py-debug

Running from node

If you want to run the normal Python CLI, you can use python.sh. It takes the same options as the normal Python CLI entrypoint, though the REPL does not function and will crash.

python.sh invokes node_entry.mjs which imports the Emscripten module for the Python process and starts it up with the appropriate settings. If you wish to make a node application that “embeds” the interpreter instead of acting like the CLI you will need to write your own alternative to node_entry.mjs.

Running tests

After building, you can run the full test suite with:

./cross-build/wasm32-emscripten/build/python/python.sh -m test -uall

You can run the browser smoke test with:

./Tools/wasm/emscripten/browser_test/run_test.sh

The Web Example

When building for Emscripten, the web example will be built automatically. It is in the web_example directory. To run the web example, cd into the web_example directory, then run python server.py. This will start a web server; you can then visit http://localhost:8000/ in a browser to see a simple REPL example.

The web example relies on a bug fix in Emscripten version 3.1.73 so if you build with earlier versions of Emscripten it may not work. The web example uses SharedArrayBuffer. For security reasons browsers only provide SharedArrayBuffer in secure environments with cross-origin isolation. The webserver must send cross-origin headers and correct MIME types for the JavaScript and WebAssembly files. Otherwise the terminal will fail to load with an error message like ReferenceError: SharedArrayBuffer is not defined. See more information here: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/SharedArrayBuffer#security_requirements

Note that SharedArrayBuffer is not required to use Python itself, only the web example. If cross-origin isolation is not appropriate for your use case you may make your own application embedding python.mjs which does not use SharedArrayBuffer and serve it without the cross-origin isolation headers.

Embedding Python in a custom JavaScript application

You can look at python.worker.mjs and node_entry.mjs for inspiration. At a minimum you must import createEmscriptenModule and you need to call createEmscriptenModule with an appropriate settings object. This settings object will need a prerun hook that installs the Python standard library into the Emscripten file system.

NodeJs

In Node, you can use the NodeFS to mount the standard library in your native file system into the Emscripten file system:

import createEmscriptenModule from "./python.mjs";

await createEmscriptenModule({
    preRun(Module) {
        Module.FS.mount(
            Module.FS.filesystems.NODEFS,
            { root: "/path/to/python/stdlib" },
            "/lib/",
        );
    },
});

Browser

In the browser, the simplest approach is to put the standard library in a zip file it and install it. With Python 3.14 this could look like:

import createEmscriptenModule from "./python.mjs";

await createEmscriptenModule({
  async preRun(Module) {
    Module.FS.mkdirTree("/lib/python3.14/lib-dynload/");
    Module.addRunDependency("install-stdlib");
    const resp = await fetch("python3.14.zip");
    const stdlibBuffer = await resp.arrayBuffer();
    Module.FS.writeFile(`/lib/python314.zip`, new Uint8Array(stdlibBuffer), {
      canOwn: true,
    });
    Module.removeRunDependency("install-stdlib");
  },
});

Limitations and issues

Network stack

• Python‘s socket module does not work with Emscripten’s emulated POSIX sockets yet. Network modules like asyncio, urllib, selectors, etc. are not available.
• Only AF_INET and AF_INET6 with SOCK_STREAM (TCP) or SOCK_DGRAM (UDP) are available. AF_UNIX is not supported.
• socketpair does not work.
• Blocking sockets are not available and non-blocking sockets don't work correctly, e.g. socket.accept crashes the runtime. gethostbyname does not resolve to a real IP address. IPv6 is not available.
• The select module is limited. select.select() crashes the runtime due to lack of exectfd support.

processes, signals

• Processes are not supported. System calls like fork, popen, and subprocess fail with ENOSYS or ENOSUP.
• Signal support is limited. signal.alarm, itimer, sigaction are not available or do not work correctly. SIGTERM exits the runtime.
• Keyboard interrupt (CTRL+C) handling is not implemented yet.
• Resource-related functions like os.nice and most functions of the resource module are not available.

threading

• Threading is disabled by default. The configure option --enable-wasm-pthreads adds compiler flag -pthread and linker flags -sUSE_PTHREADS -sPROXY_TO_PTHREAD.
• pthread support requires WASM threads and SharedArrayBuffer (bulk memory). The Node.JS runtime keeps a pool of web workers around. Each web worker uses several file descriptors (eventfd, epoll, pipe).
• It's not advised to enable threading when building for browsers or with dynamic linking support; there are performance and stability issues.

file system

• Most user, group, and permission related function and modules are not supported or don't work as expected, e.g.pwd module, grp module, os.setgroups, os.chown, and so on. lchown and lchmod are not available.
• umask is a no-op.
• hard links (os.link) are not supported.
• Offset and iovec I/O functions (e.g. os.pread, os.preadv) are not available.
• os.mknod and os.mkfifo don't work and are disabled.
• Large file support crashes the runtime and is disabled.
• mmap module is unstable. flush (msync) can crash the runtime.

Misc

• Heap memory and stack size are limited. Recursion or extensive memory consumption can crash Python.
• Most stdlib modules with a dependency on external libraries are missing, e.g. ctypes, readline, ssl, and more.
• Shared extension modules are not implemented yet. All extension modules are statically linked into the main binary. The experimental configure option --enable-wasm-dynamic-linking enables dynamic extension support. It's currently known to crash in combination with threading.
• glibc extensions for date and time formatting are not available.
• locales module is affected by musl libc issues, gh-90548.
• Python's object allocator obmalloc is disabled by default.
• ensurepip is not available.

In the browser

• The interactive shell does not handle copy 'n paste and unicode support well.
• The bundled stdlib is limited. Network-related modules, multiprocessing, dbm, tests and similar modules are not shipped. All other modules are bundled as pre-compiled pyc files.
• In-memory file system (MEMFS) is not persistent and limited.
• Test modules are disabled by default. Use --enable-test-modules build test modules like _testcapi.

WASI (wasm32-wasi)

See the devguide on how to build and run for WASI.

Detecting WebAssembly builds

Python code

import os, sys

if sys.platform == "emscripten":
    # Python on Emscripten
    ...
if sys.platform == "wasi":
    # Python on WASI
    ...

if os.name == "posix":
    # WASM platforms identify as POSIX-like.
    # Windows does not provide os.uname().
    machine = os.uname().machine
    if machine.startswith("wasm"):
        # WebAssembly (wasm32, wasm64 potentially in the future)

>>> import os, sys
>>> os.uname()
posix.uname_result(
    sysname='Emscripten',
    nodename='emscripten',
    release='4.0.2',
    version='#1',
    machine='wasm32'
)
>>> os.name
'posix'
>>> sys.platform
'emscripten'
>>> sys._emscripten_info
sys._emscripten_info(
    emscripten_version=(3, 1, 10),
    runtime='Mozilla/5.0 (X11; Linux x86_64; rv:104.0) Gecko/20100101 Firefox/104.0',
    pthreads=False,
    shared_memory=False
)

>>> sys._emscripten_info
sys._emscripten_info(
    emscripten_version=(3, 1, 19),
    runtime='Node.js v14.18.2',
    pthreads=True,
    shared_memory=True
)

>>> import os, sys
>>> os.uname()
posix.uname_result(
    sysname='wasi',
    nodename='(none)',
    release='0.0.0',
    version='0.0.0',
    machine='wasm32'
)
>>> os.name
'posix'
>>> sys.platform
'wasi'

C code

Emscripten SDK and WASI SDK define several built-in macros. You can dump a full list of built-ins with emcc -dM -E - < /dev/null and /path/to/wasi-sdk/bin/clang -dM -E - < /dev/null.

• WebAssembly __wasm__ (also __wasm)
• wasm32 __wasm32__ (also __wasm32)
• wasm64 __wasm64__
• Emscripten __EMSCRIPTEN__ (also EMSCRIPTEN)
• Emscripten version __EMSCRIPTEN_major__, __EMSCRIPTEN_minor__, __EMSCRIPTEN_tiny__
• WASI __wasi__