dart_monty #

Live Demo | GitHub | Monty

Monty is a restricted, sandboxed Python interpreter built in Rust by the Pydantic team. It runs a safe subset of Python designed for embedding.

dart_monty provides pure Dart bindings for the Monty interpreter, bringing sandboxed Python execution to Dart and Flutter apps — on desktop, web, and mobile — with resource limits, iterative execution, and snapshot/restore support.

Platform Support #

Installation #

flutter pub add dart_monty

Usage #

import 'package:dart_monty/dart_monty.dart';

// Simple execution
final monty = MontyPlatform.instance;
final result = await monty.run('2 + 2');
print(result.value); // 4

// With resource limits
final limited = await monty.run(
  'fib(30)',
  limits: MontyLimits(timeoutMs: 5000, memoryBytes: 10 * 1024 * 1024),
);

// Iterative execution (external functions)
var progress = await monty.start(
  'fetch("https://example.com")',
  externalFunctions: ['fetch'],
);

if (progress is MontyPending) {
  print('Python called: ${progress.functionName}');
  progress = await monty.resume(myResult);
}

final complete = progress as MontyComplete;
print(complete.result.value);

// Error injection
progress = await monty.resumeWithError('network timeout');

// Async/futures — concurrent external calls via asyncio.gather
progress = await monty.start('''
import asyncio

async def main():
  a, b = await asyncio.gather(fetch("url1"), fetch("url2"))
  return a + b

await main()
''', externalFunctions: ['fetch']);

// Return Future for each pending call (native only)
while (progress is MontyPending) {
  progress = await monty.resumeAsFuture();
}

// Resolve all pending futures at once
if (progress is MontyResolveFutures) {
  final results = await Future.wait([fetchUrl("url1"), fetchUrl("url2")]);
  progress = await monty.resolveFutures({
    0: results[0],
    1: results[1],
  });
}

// Cleanup
await monty.dispose();

Monty API Coverage (~35%) #

dart_monty wraps the upstream Monty Rust API. The table below shows current coverage and what's planned.

Architecture #

Federated plugin with six packages:

The three pure-Dart packages can be used without Flutter (e.g. in CLI tools or server-side Dart).

Native Path (desktop) #

Dart app -> MontyDesktop (Isolate)
  -> MontyFfi (dart:ffi)
    -> libdart_monty_native.{dylib,so}
      -> Monty Rust interpreter

Web Path (browser) #

Dart app (compiled to JS) -> DartMontyWeb
  -> MontyWasm (dart:js_interop)
    -> Web Worker -> @pydantic/monty WASM

The Web Worker architecture bypasses Chrome's 8 MB synchronous WASM compilation limit.

Web Setup #

The web backend requires COOP/COEP HTTP headers for SharedArrayBuffer support:

Cross-Origin-Opener-Policy: same-origin
Cross-Origin-Embedder-Policy: require-corp

Contributing #

See CONTRIBUTING.md for development setup, gate scripts, and CI details.

License #

MIT License. See LICENSE.