dart_monty

Sandboxed Python interpreter for Dart and Flutter. Run Python from native, web, and mobile — one package, every platform.

FFI (Native) WASM (Browser) REPL Plugins Sandbox

Quick Start

Three API levels — from one-shot execution to full plugin sessions.

One-Shot Execution

// Run Python and get the result
final result = await Monty.exec('2 + 2');
print(result.value); // MontyInt(4)

Stateful REPL

// Variables, functions, classes persist across calls
final repl = MontyRepl();
await repl.feed('x = 42');
await repl.feed('def double(n): return n * 2');
final r = await repl.feed('double(x)');
print(r.value); // MontyInt(84)

Session with Plugins

// Full plugin dispatch — template, messaging, sandboxing
final session = ReplSession(
  plugins: [
    DinjaTemplatePlugin(),
    MessageBusPlugin(),
    SandboxPlugin(platformFactory: () async => MontyFfi()),
  ],
);

final r = await session.run(
  "tmpl_render(template='Hello {{ name }}!', context={'name': 'World'})",
);
print(r.value); // 'Hello World!'

REPL — Native Heap Persistence

Functions, classes, closures, and heap objects survive across calls. No JSON serialization — the Rust interpreter maintains state natively.

MontyRepl

Low-level REPL with feed() and feedStart()/resume() for host function dispatch.

ReplSession

High-level session combining REPL + plugin dispatch + event streaming. The recommended API.

Continuation Detection

Detects incomplete input (>>> vs ... prompts) for building REPL UIs.

Try the interactive REPL demo →   |   REPL documentation →

Built-in Plugins

Host functions that Python code can call. All plugins work with both ReplSession and AgentSession.

TemplatePlugin

Jinja2 template rendering. Variables, loops, conditionals, filters.

tmpl_render(template, context)

MessageBusPlugin

In-memory named message channels. Send, receive, peek, stats.

msg_send · msg_recv · msg_peek · msg_close · msg_stats

SandboxPlugin

Spawn isolated child interpreters. Parallel execution, gather, plugin inheritance, grandchildren.

sandbox_spawn · sandbox_await · sandbox_gather · sandbox_free

OS / Filesystem

Configurable OS call providers for filesystem, time, and environment:

In-Memory VFS

Read/write filesystem in memory. No host disk access.

Read-Only Mount

Mount a host directory read-only into the sandbox.

Overlay

Copy-on-write layer over a read-only base. Writes go to memory.

Plugin documentation →   |   Sandbox deep dive →

Architecture

One package. Conditional imports select FFI or WASM at compile time.

dart_monty (single package) ├── lib/src/platform/ Core types: MontyResult, MontyProgress, MontyValue ├── lib/src/ffi/ Native FFI backend (dart:ffi) ├── lib/src/wasm/ Web WASM backend (dart:js_interop + Worker) ├── lib/src/bridge/ Plugin dispatch, events, middleware ├── lib/src/repl/ REPL: MontyRepl, ReplSession, ReplPlatform └── native/ Rust C API crate (compiles to .dylib/.so/.dll + .wasm)

Execution Paths

Native (FFI)

Dart → dart:ffi → libdart_monty_native → Monty Rust interpreter

Desktop, server, mobile

Web (WASM)

Dart/JS → js_interop → Web Worker → Monty WASM

Browser (any modern browser)

Bridge & Events

DefaultMontyBridge orchestrates execution. Plugins register host functions. When Python calls one, execution suspends, the bridge dispatches to the handler, and resumes with the result. Every step emits BridgeEvents for real-time UI updates.

Full architecture docs →

Interactive Demos

All demos run in your browser via WebAssembly. No server required.

REPL

Interactive Python REPL with real plugins (template, message bus).

Basic Execution

One-shot Python execution, error handling, external functions.

Virtual Filesystem

Python pathlib operations against in-memory VFS.

Agent Session

Stateful agent with plugins, telemetry, and event streaming.

Python Ladder

350+ compatibility tests across 21 tiers.

Sorting Visualizer

Step-by-step sorting algorithm animations in Python.

Get Started

Install

dart pub add dart_monty

Native (FFI)

Works out of the box on macOS, Linux, and Windows. The Rust native library is built automatically via Dart's native assets hook.

Web (WASM)

Copy the WASM assets to your web directory:

cp packages/dart_monty_wasm/assets/dart_monty_bridge.js web/
cp packages/dart_monty_wasm/assets/dart_monty_worker.js web/
cp packages/dart_monty_wasm/assets/dart_monty_native.wasm web/

Serve with Cross-Origin-Opener-Policy: same-origin and Cross-Origin-Embedder-Policy: require-corp headers (required for SharedArrayBuffer).

Documentation