Comment by underdeserver

To be fair, even if there is/were a team, I don’t know that writing a new backend from scratch would be a good use of their time. pytype apparently started before mypy or any of the other Python type checkers existed. [1] But at this point there’s mypy, pyright, pyre/pyrefly, Ty, and probably more I’m not thinking of. It sounds more useful to collaborate with one of those existing projects than to write yet another new type checker.

Especially when, in my experience, each checker produces slightly different results on the same code, effectively creating its own slightly different language dialect with the associated fragmentation cost. In theory that cost could be avoided through more rigorous standardization efforts to ensure all the checkers work exactly the same way. But that would also reduce the benefit of writing a new type checker, since there would be less room to innovate or differentiate.

[1] https://news.ycombinator.com/item?id=19486938

there is a new python team, we met up with them at pycon and had some nice conversations. as a former pytype dev I will be the first to admit that maintaining it as a legacy project without the context of having developed it over the years would not have been a pleasant experience at all, but also pytype, while very powerful at what it did, definitely had some flaws that put it firmly in the last generation of type checkers.

the current generation (mostly ty and pyrefly right now, though major props to pyright for being ahead of the curve) is moving towards fast, incremental type checking with LSP integration, and pytype was never going to get there. it's fundamentally a slow, batch-based type checker, which will catch a lot of errors in your project, but which will never be usable as an incremental type checker within your ide. add that to the fact that it had a different philosophy of type checking from most of the other major checkers and you had users facing the issue that their code would be checked one way by pyright in the ide, and then a subtly different way by pytype in the CI pipeline.

I loved my time working on pytype, and I would like to see some of its features added to pyrefly, but it has definitely been superseded by now.

There is still a team within Google in charge of this space.

> Is there a good reason to avoid using Pyrefly?

Wouldn't the other way around be easier for finding good tools? Figure out what matters to you, inspect if the project fulfills those needs and then go with it after making sure it works well for you.

Regardless, a comparison between the two was posted to HN not too long time ago: https://news.ycombinator.com/item?id=44107655

(Pyrefly dev here) As another commenter mentioned, Pyrefly is still in alpha. Sorry we don't make that more clear!

While we are in alpha, and there are plenty of open issues we are still working through, I think Pyrefly is actually pretty usable already, especially for code navigation.

https://github.com/facebook/pyrefly/releases

Pyrefly v0.29.0

Status : ALPHA

I believe Pyrefly is stricter, so it may be a better choice for new projects but harder to integrate into existing ones without type-checking.

The cost of your dogmatic preference is your Python experience being more miserable than it should be. Astral's ruff and uv are widely adopted for a good reason, and there is no reason to think that ty will come any different.