monty-type-checking

Type checking for Monty, powered by ty.

Monty supports full modern Python type hints. This crate embeds ty's semantic analysis (Astral's ty_python_semantic engine — the same one behind the ty type checker) and checks code against monty-typeshed: a trimmed typeshed describing the stdlib subset Monty actually implements. Code that uses unsupported stdlib surface is therefore flagged before it runs rather than failing at runtime.

It backs monty --type-check in the CLI and the type_check option on sessions in the pydantic-monty and @pydantic/monty packages.

Usage

use monty_type_checking::{SourceFile, type_check};

let source = SourceFile::new("x: int = 'not an int'", "main.py");
let diagnostics = type_check(&source, None).unwrap();
// `Some(...)` means typing errors were found; `None` means the code is clean
assert!(diagnostics.is_some());

The second argument is an optional stubs file declaring names the host will provide at runtime (external functions, inputs). The stubs are written alongside the source and a from <stubs> import * line is injected, so checked code can reference host functions without defining them — diagnostic line numbers are adjusted back to the original source.

TypeCheckingDiagnostics renders ty's full diagnostic output (source context, underlines, and optional ANSI color) via its Display implementation.

Pooled databases

Each check leases a pre-configured in-memory salsa database from a small process-wide pool instead of rebuilding typeshed-derived semantic state per call. Leased databases are scrubbed of the checked files and returned to the pool when the check (or the diagnostics value it produced) is dropped, keeping salsa's single-writer invariant while allowing concurrent checks on different databases.

Monty crates

• monty — the core interpreter: Python parser, bytecode VM, and sandbox.
• monty-types — the shared boundary data types (values, exceptions, OS calls, resource limits) hosts use without linking the interpreter.
• monty-fs — host-side filesystem mounts: maps virtual sandbox paths to real host directories.
• monty-runtime — the monty binary: REPL, file runner, and subprocess worker mode.
• monty-pool — an elastic pool of crash-isolated monty worker subprocesses.
• monty-proto — the protobuf wire protocol spoken between pool parents and workers.
• monty-type-checking — type checking of sandboxed code, powered by ty. this crate
• monty-typeshed — the trimmed typeshed stubs describing the stdlib subset Monty implements.
• monty-macros — the proc macros behind monty's argument parsing.