Meta Tech Podcast15 Mai 2025

75: Open-sourcing Pyrefly - A faster Python type checker written in Rust

Pyrefly is a faster, open-source Python type checker written in Rust, succeeding Pyre. But what prompted the rewrite and what besides the language choice ended up making it faster? Host Pascal talks to Maggie, Rebecca and returning guest Neil about the unexpected complexities of building an incremental type checker that scales to mono repositories in episode 75.

Got feedback? Send it to us on Threads (https://threads.net/@metatechpod), Instagram (https://instagram.com/metatechpod) and don't forget to follow our host Pascal (https://mastodon.social/@passy, https://threads.net/@passy_). Fancy working with us? Check out https://www.metacareers.com/.

Links

Pyrefly: https://pyrefly.org/
Pyre: https://pyre-check.org/
Ruff: https://github.com/astral-sh/ruff
PEP 484: https://peps.python.org/pep-0484/

Timestamps

Intro 0:06
Rebecca Introduction 1:45
Maggie Introduction 2:45
Neil (Re-)Introduction 3:12
Team Mission 3:56
History of Typing in Python 4:29
The State of Typed Python at Meta 5:32
fbcode 6:02
Original Motivation for building Pyre 6:19
Justifying the Rewrite 7:48
Pyrefly vs the Rest 9:41
Why Rust? 10:45
Fearless Concurrency 12:02
Why is it faster? 12:37
Python community and Rust 14:57
Pyrefly wasm crate 15:46
Upgrade experience 17:34
Type checking differences 19:12
IDE experience 21:31
State of Pyrefly at Meta 22:27
Being open-source-first 23:36
Open-source challenges 25:06
Unexpected challenges 26:39
Outro 31:05

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