cpython-wasm

CPython 3.13 compiled to WebAssembly (wasm32-wasi). Run Python in the browser with a real CPython interpreter — no server required.

Install

npm install cpython-wasm

Usage

import { PythonRuntime } from 'cpython-wasm';

// Zero config — automatically downloads python.wasm and stdlib from GitHub Releases
const py = new PythonRuntime();

py.onStdout = (line) => console.log(line);
py.onStderr = (line) => console.error(line);

await py.init(); // downloads ~33 MB on first run

await py.run('print("Hello from Python!")');
await py.run('import sys; print(sys.version)');
await py.run(`
result = sum(i**2 for i in range(10))
print(f"Sum of squares: {result}")
`);

py.destroy();

Custom URLs (optional)

Serve files locally for faster load times:

const py = new PythonRuntime({
  wasmUrl: '/assets/python.wasm',
  stdlibUrl: '/assets/python313-stdlib.zip',
});

Download from GitHub Releases.

API

new PythonRuntime(opts?)

| Option | Type | Default | |--------|------|---------| | wasmUrl | string | GitHub Releases | | stdlibUrl | string | GitHub Releases | | workerUrl | string? | auto |

py.init(): Promise<void>

Load stdlib, compile WASM, initialize runtime. Must be called before run().

py.run(code: string): Promise<RunResult>

Execute Python code. Returns when execution completes.

py.writeFile(path, content)

Write a file into the virtual filesystem.

py.destroy()

Terminate the worker and free resources.

py.onStdout = (line: string) => void

py.onStderr = (line: string) => void

Requirements

• Browser with WebAssembly + Web Workers support
• Internet access for first load (or self-host the files)

How it works

CPython 3.13 is compiled to wasm32-wasi using wasi-sdk. A custom WASI shim implements the system interface (file I/O, clocks, random) using browser APIs. The stdlib is extracted from a zip into an in-memory virtual filesystem on startup.

License

MIT