alt.hn

3/16/2026 at 12:25:18 PM

Comparing Python Type Checkers: Typing Spec Conformance

 https://pyrefly.org/blog/typing-conformance-comparison/

by ocamoss

3/16/2026 at 5:42:44 PM

Wow, quite surprising results. I have been working on a personal project with the astral stack (uv, ruff, ty) that's using extremely strict lint/type checking settings, you could call it an experiment in setting up a python codebase to work well with AI. I was not aware that ty's gaps were significant. I just tried with zuban + pyright. Both catch a half dozen issues that ty is ignoring. Zuban has one FP and one FN, pyright is 100% correct.

Looks like I will be converting to pyright. No disrespect to the astral team, I think they have been pretty careful to note that ty is still in early days. I'm sure I will return to it at some point - uv and ruff are excellent.

by extr

3/16/2026 at 6:17:03 PM

Using VSCodium I was having issues with python type checkers for quite a while. I did the basedpyright thing for a while but that was painful. It's a bit too based for me, and I'm not sure i'd call it based. Right now I have uv, ruff, and ty and I'm happy with it. It's super easy to update and super fast. I didn't realize the coverage wasn't as good as some others but I still like it. I may have to try pyrefly. Never heard of it until this post, so thank you.

by pgwalsh

3/16/2026 at 2:57:39 PM

I've been using ty on some previously untyped codebases at work. It does a good job of being fast and easy to use while catching many issues without being overly draconian.

My teammates who were writing untyped Python previously don't seem to mind it. It's a good addition to the ecosystem!

by martinky24

3/16/2026 at 3:38:23 PM

And it makes it infinitely easier for them to get with the times and start typing their code!

by tfrancisl

3/16/2026 at 3:48:48 PM

I am worried about the false negatives/positive rate however. Hope it improves.

by rirze

3/16/2026 at 4:26:50 PM

My understand is Astral's focus for ty has been on making a good experience for common issues, whereas they plan for very high compliance but difficult or rare edge cases aren't are prioritized.

Compliance suite numbers are biased towards edge cases and not the common path because that's where a lot of the tests need to be added.

My advise is to see how each type checker runs against your own codebase and if the output/performance is something you are happy with.

by notatallshaw

3/16/2026 at 5:22:56 PM

> My understand is Astral's focus for ty has been on making a good experience for common issues, whereas they plan for very high compliance but difficult or rare edge cases aren't are prioritized.

I would say that's true in terms of prioritization (there's a lot to do!), but not in terms of the final user experience that we are aiming for. We're not planning on punting on anything in the conformance suite, for instance.

by dcreager