Meta Tech Podcast
75: Open-sourcing Pyrefly - A faster Python type checker written in Rust
May 15, 2025
Explore the fascinating world of Pyrefly, a groundbreaking type checker for Python built with Rust. Discover the motivations behind its creation and how it dramatically improves performance and developer experience. The hosts delve into the historical evolution of Python's type system, highlighting a cultural shift towards embracing diverse programming languages. Learn about the challenges and innovations in building an incremental type checker, and peek into the future of type checking with exciting tools and community engagement.
32:22
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Quick takeaways
- Pyrefly, built in Rust, significantly improves type checking speed for Python, enabling checks of up to 1.8 million lines of code per second.
- The project prioritizes user experience by easing transitions from other type checkers and engaging the open-source community for continuous improvement.
Deep dives
Introduction of Pyrefly and its Purpose
Meta has released Pyrefly, an alpha type checker for Python, aiming to enhance the developer experience in the rapidly expanding machine learning area where Python is predominant. The motivation behind creating Pyrefly stems from the limitations observed in existing type checkers, particularly regarding speed and effectiveness, especially as codebases grow larger. With a focus on ensuring faster type checking and improved reliability, Pyrefly is designed to better accommodate the needs of Python developers. The developers highlighted the goal of moving beyond mere adequacy to providing a delightful experience, which necessitated a complete rewrite of their previous type checker, Pyre.
