Google Replaces pytype With Pyrefly: Which Type Checker in 2026

Google has announced it will replace its in-house Python type checker, pytype, with Meta's Rust-based type checker Pyrefly. The change was announced on October 6, 2026. Pyrefly is reportedly an order of magnitude faster than Pytype on internal benchmarks from Google, and scores 97% on the official typing conformance suite.

The announcement marks the end of support for pytype, as its README now states that Python 3.12 is the last supported version. Pyrefly is positioned as a drop-in replacement for pytype and mypy users looking for speed, with a tool called pyrefly init to convert mypy or Pyright configs. Ty, from Astral, is also fast, but it is in beta with no stable API, so it is recommended as something to trial rather than gate releases on.

Google's switch to Pyrefly is driven by three main factors: speed, conformance to new Python versions and typing PEPs, and improved diagnostics. Pyrefly's errors point directly at the offending code with inline annotations and sometimes suggest a fix. For example, Pyrefly would reject a function that takes an int but is called with an int | None, suggesting a narrowing check with an is not None.

The migration path suggests installing Pyrefly, running pyrefly init to convert existing config files, checking locally, suppressing errors, adding Pyrefly to CI beside the old checker, and then removing the old type checker. The announcement does not provide a deadline for the switch, but it recommends planning the migration now, especially when planning to upgrade to Python 3.13 or later, as pytype will no longer work on those versions.