An Update on Pytype
github.com72 points by mxmlnkn 2 hours ago
72 points by mxmlnkn 2 hours ago
ex-pytype dev here - we knew this was coming and it's definitely the right thing to do, but it's still a little sad to see the end of an era. in particular, pytype's ability to do flow-based analysis across function boundaries (type checking calls to unannotated functions by symbolically executing the function body with the types of the call arguments) has not been implemented by any of the other checkers (again for good reasons; it's a performance hit and the world is moving towards annotations over pure inference anyway, but I still think it's a nice feature to have and makes for more powerful checking).
as an aside, while I agree that bytecode-based analysis has its drawbacks, I think it's a tool worth having in the overall python toolbox. I spun off pycnite from pytype in the hope that anyone else who wanted to experiment with it would have an easier time getting started - https://github.com/google/pycnite
I have recently jumped onto the "write python tooling in rust" bandwagon and might look into a rust reimplementation of pycnite at some point, because I still feel that bytecode analysis lets you reuse a lot of work the compiler has already done for you.
I think this is for the best.
I used Pytype at Google years ago and while it's well written and the team was responsive, ultimately Python is not well suited for type checking Python. It's compute intensive.
I think the Ty people at Astral have the correct idea, and hope it'll work out.
In theory, nothing prevents the pytype team at Google to develop a new backend in a different language.
In practice, there is no longer a pytype team at Google [https://news.ycombinator.com/item?id=40171125], which I suspect is the real reason for the discontinuation.
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.
I've heard of `ty` too but recently I learned about Pyrefly, which is not in pre-production alpha, and is also Rust: https://pyrefly.org/
Is there a good reason to avoid using Pyrefly?
> 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
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.
in the related FAQ https://github.com/google/pytype/issues/1925 they point explicitly to the future:
> What alternatives can I consider? There are four Python static type checkers that can be considered: mypy and Pyright have been released to the community for a while and have well established user bases. Pyrefly, ty were announced recently at PyCon US 2025 and are in active development stage in the current time of August 2025 when this was written.
mypy - https://github.com/python/mypy
Pyright - https://github.com/microsoft/pyright
Pyrefly - https://github.com/facebook/pyrefly
I'm surprised Google still maintained their own solution for this for so long. The standard for statically type checking Python nowadays is mypy.
Mypy is far too slow to type check a codebase like Google. That's why Facebook, Google, and Microsoft have/had their own solutions.
Google, Facebook, and Microsoft all maintain(ed) independent non-mypy typecheckers for internal and external uses that aren't served by mypy.
The various features mypy didn't support include speed, type inference/graduality, and partial checking in the presence of syntax errors (for linter/interactive usecases and code completion).
Maybe they could do typechecking using an LLM agent? I'm sure they'd fund a team for that.
astral bags another one
Is ty more mature than pyright or mypy?
I'm currently using pyright, but I'm going to migrate once ty and its vscode extension are given the "production ready" greenlight.
at this stage I get very few false positives and it's so much easier to configure and use than pyright
Free fire 2018
Another abandoned project from Google? Not surprised. Never trust on Google.
Pytype is used heavily inside Google so they bear the penalty likely more than you. Besides, py typing libraries is a dynamically changing landscape so it isn't anything out of the norm. Not everything is an abandoned project, and if anything Google abandons some projects well after the winners and losers are apparent eg Tensorflow.
Pytype was cool before Python type annotations became widespread. It seems to me like the industry is naturally moving toward native type annotations and linters and away from static analyzers like Pytype and mypy.
Pytype and mypy check native annotations.
Well yes but with native annotations the linter you’re already using can do a lot of the type checking work so for many teams it’s not worth it to add Pytype or mypy