Meta open-sources Pyrefly, its Rust-based Python type checker

Meta has made Pyrefly — a Python type checker and IDE tool written in Rust — available to the open-source community. The project traces back to 2017, when Meta engineers set out to build a type checker for Instagram’s typed Python codebase; as the language’s type system evolved over the years, that tool eventually became Pyrefly.

Pyrefly supports both command-line usage and IDE integration, and is intended to catch errors before runtime in Python projects of any size. The team behind it highlights its incremental design, which scales to monorepo-sized codebases.

On the latest episode of the Meta Tech Podcast, host Pascal Hartig speaks with Pyrefly engineers Maggie, Rebecca, and Neil about the release and the engineering behind an incremental type checker that handles large-scale repositories.

The episode is available on Spotify, Apple Podcasts, and Pocket Casts, or wherever you get your podcasts.

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