Pyrefly: A new type checker and IDE experience for Python

Summary

Today we are announcing an alpha version of Pyrefly, an open source Python type checker and IDE extension crafted in Rust. Pyrefly is a static type checker that analyzes Python code to ensure type consistency and help you catch errors throughout your codebase before your code runs. It also supports IDE integration and CLI usage to give you flexibility in how you incorporate it into your workflow. The open source community is the backbone of the Python language. We are eager to collaborate on Pyrefly with the community and improve Python’s type system and the many libraries that we all rely on. Get started Ready to dive in? The official Pyrefly website has all the details, but to quickly get started: Why we built Pyrefly Back in 2017, we embarked on a mission to create a type checker that could handle Instagram’s massive codebase of typed Python. This mission led to the birth of the Pyre type checker, inspired by the robust designs of Hack and Flow, and written in OCaml to deliver scalable performance. Over the years, Pyre served us well, but as the type system evolved and the need for typechecking to drive responsive IDE emerged, it was clear that we needed to take a new approach. We explored alternate solutions and leveraged community tools like Pyright for code navigation. But the need for an extensible type checker that can bring code navigation, checking at scale, and exporting types to other services drove us to start over, creating Pyrefly. The principles behind Pyrefly Today, we’re excited to unveil Pyrefly, a project we’ve been developing openly on GitHub. We invite you to explore our work and try it out on your own project. While a project like Pyrefly is the sum of thousands of technical choices, a few notable principles we’ve followed are: Performance We want to shift checks that used to happen later on CI to happening on every single keystroke. That requires checking code at speed (on large codebases we can check 1.8 million lines of code per second!) an...