Pyrefly v1.2 is now available 🎉 This release adds support for Bazel and attrs, along with improvements to type checking, the language server, and the CLI. This release includes 901 commits from 59 contributors. To get started, run: pip ins…
Companion post to my PyCon 2026 Typing Summit talk, "From Soundness to Blame: AI-Assisted Formalization of a Tiny Python." This is the first in a series. What this series is about Python's type system is practical, useful, but full of edge…
Pyrefly v1.1 is now available 🎉 This is our first minor release since reaching stable v1.0 in May, and it brings improvements across the board: better performance, smarter type checking, and new IDE refactoring tools. This release bundles
Why aren't tensor shapes part of the type system? When you write a PyTorch model, the hardest part of composing tensor ops is keeping track of their shapes. The standard practice today is to write shapes down as comments as you go. This tal…
Mypy, Pyrefly, Pyright, ty, Zuban, and possibly more that will come in the future... how are library maintainers expected to cope? TL;DR: Prioritise running as many type-checkers as possible on your test suite. Run at least one on your sour…
Dify is a large open-source platform for building LLM applications. Its backend is a Python Flask application with workflows, RAG pipelines, model providers, agents, Celery tasks, database migrations, and a large test suite. That makes it a…
Today we are pleased to share that Pyrefly, our open source type checker and language server for Python, has reached stable version 1 status, meaning we are confident that Pyrefly is ready for production use. Pyrefly is a Python code analys…
Coding agents are writing more Python than ever. Tools like Claude, Copilot, Cursor, and Codex generate entire features with little-to-no user interaction. But in large projects, this generated code is prone to type errors, mismatched signa…
We frequently hear from developers who are excited about the new generation of checkers (Ty and Pyrefly) and want to know how they stack up against each other and the existing, established tools (Mypy and Pyright). In this comparison, we'll…
At Pyrefly, we've always believed that type coverage is one of the most important indicators of code quality. Over the past year, we've worked closely with teams across large Python codebases here at Meta - improving performance, tightening…
Pyrefly is a next-generation Python type checker and language server, designed to be extremely fast and featuring advanced refactoring and type inference capabilities. This isn’t the Pyrefly team’s first time building a type checker for Pyt…
When you write typed Python, you expect your type checker to follow the rules of the language. But how closely do today's type checkers actually follow the Python typing specification? In this post, we look at what typing spec conformance m…