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Open-sourcing Pyrefly: A faster Python type checker written in Rust

Back in 2017, engineers at Meta sought to create a type checker for Instagram’s typed Python codebase. Years later, as the type system continued to evolve, that type checker eventually became Pyrefly. Pyrefly is a new type checker and IDE experience for Python, written with Rust, and now available for the entire Python community to [...] Read More... The post Open-sourcing Pyrefly: A faster Python type checker written in Rust appeared first on Engineering at Meta.

May 16, 2025 - 02:32

Open-sourcing Pyrefly: A faster Python type checker written in Rust

Back in 2017, engineers at Meta sought to create a type checker for Instagram’s typed Python codebase. Years later, as the type system continued to evolve, that type checker eventually became Pyrefly.

Pyrefly is a new type checker and IDE experience for Python, written with Rust, and now available for the entire Python community to use! It’s open-source, supports both CLI usage and IDE integration. and is designed to help you catch errors before runtime in Python codebases of any size.

On this episode of the Meta Tech Podcast, Pascal Hartig sits down with Maggie, Rebecca, and Neil — some of the team behind Pyrefly — to discuss this latest release from Meta and how they built an incremental type checker that scales to mono repositories.

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The Meta Tech Podcast is a podcast, brought to you by Meta, where we highlight the work Meta’s engineers are doing at every level – from low-level frameworks to end-user features.

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