Ty: A fast Python type checker and LSP

Summary

TL;DR: ty is an extremely fast Python type checker and language server, written in Rust, and designed as an alternative to tools like mypy, Pyright, and Pylance. Today, we're announcing the Beta release of ty. We now use ty exclusively in our own projects and are ready to recommend it to motivated users for production use. At Astral, we build high-performance developer tools for the Python ecosystem. We're best known for uv, our Python package manager, and Ruff, our linter and formatter. Today, we're announcing the Beta release of the next tool in the Astral toolchain: ty, an extremely fast Python type checker and language server, written in Rust. 0s20s40styPyreflyPyrightmypy5.32s19.62s45.66s2.19sType checking the home-assistant project on the command-line, without caching (M4). ty was designed from the ground up to power a language server. The entire ty architecture is built around "incrementality", enabling us to selectively re-run only the necessary computations when a user (e.g.) edits a file or modifies an individual function. This makes live updates extremely fast in the context of an editor or long-lived process. 0s1s2styPyrightPyrefly370.5ms2.60s4.5msRe-computing diagnostics in the language server after editing a file in the PyTorch project (M4). You can install ty today with uv tool install ty@latest, or via our VS Code extension. Like Ruff and uv, ty's implementation was grounded in some of our core product principles: An obsessive focus on performance. Without caching, ty is consistently between 10x and 60x faster than mypy and Pyright. When run in an editor, the gap is even more dramatic. As an example, after editing a load-bearing file in the PyTorch repository, ty recomputes diagnostics in 4.7ms: 80x faster than Pyright (386ms) and 500x faster than Pyrefly (2.38 seconds). ty is very fast! Correct, pragmatic, and ergonomic. With features like first-class intersection types, advanced type narrowing, and sophisticated reachability analysis, ty pushes forw...