Happy Path Programming #115 More Python Type Checking! Pyrefly with Aaron Pollack & Steven Troxler
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Aug 11, 2025 Aaron Pollack and Steven Troxler from Meta discuss the exciting evolution of Python type checking, focusing on their innovative tool, Pyrefly—a Rust-based type checker with IDE integration. They explore the journey of enhancing code quality, the growing acceptance of type annotations in the community, and the benefits of type checking for bug identification. The duo also highlights the complexities of managing type definitions within existing codebases and the potential of autotyping and AI in streamlining development processes.
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Pyre Began As Security Tool
- Pyre began at Meta as a taint-analysis tool that needed type inference to track call targets across large codebases.
- Steven Troxler describes how that security-origin project evolved into an OCaml-based type checker and later an open-source system.
Rewrite To Rust For Low Latency
- Meta rewrote Pyre into Pyrefly in Rust to reduce latency and enable a true IDE experience.
- Steven Troxler highlights Rust's native threading as a key reason for better incremental performance and lower latency.
Many Type Checkers Coexist
- The Python type-checker landscape includes MyPy, Pyrite, Pyre/Pyrefly, TY and newer entrants like Zuban, each with trade-offs.
- Teams choose based on performance, IDE integration, licensing and how well the checker scales to large codebases.