algoneer VS mypy

I gave a presentation on type checking at the EuroPython 2017 where I also investigated how many Python projects really use type checking, I present the results at the end (35 minutes in):

https://www.youtube.com/watch?v=vM2Zoy4Sxhk&t=2181s

My conclusion was that only a small fraction of projects really used them, there were a lot of projects that had type checks in their code but only in "homeopathic" doses.

I started using them for some of my Python projects as well (e.g https://github.com/algoneer/algoneer) and while I find them useful I think they're not as useful as a "real" type system in a fully typed language like Golang. Still, they're very useful for discovering simple mistakes that would only show up in unit testing otherwise.

You can also "misuse" them for other purposes, at the end of the presentation I e.g. show how you can implement software contracts with them. Of course this would wreck a type checker like mypy, so don't do that in your codebase. That's probably also one of my critiques as the annotation syntax can in principle be used for anything, but mypy and other tools are not able to deal with code that does that.

It won't have a runtime effect but if you violate it in your code somewhere it will cause static analysis tools such as Mypy to throw errors.

It looks like there's a PEP for that already! [0]

And as (almost) always, Pyright has already implemented the provisional PEP!

[0] https://github.com/python/mypy/issues/4441

They don't work in the way you would think for dataclasses or at all for dicts/TypedDicts.

See this for dataclasses: https://github.com/python/mypy/issues/5374#issuecomment-8841....

It's worth noting that Python 3.5+ added standardized type annotations for 3rd party type checkers to leverage. I pretty much use them all the time now except for extremely small scripts. My 3rd party type checker of choice is mypy.

how, when mypy itself has only "type: ignore" as a workaround for many scenarios?

here's a list of 41 issues where either the user or the mypy devs are saying "use type: ignore as a workaround for now":

https://github.com/python/mypy/issues?q=is%3Aopen+label%3Afa...

It reminds me of this 5 year old bug in mypy: "int is not a Number?" [1].

[1]: https://github.com/python/mypy/issues/3186

Mypy is an optional static type checker that aims to combine the benefits of dynamic (or “duck”) typing and static typing. The benefits of statically typed code are increased readability and maintainability. For an existing code base here's a guide on how to implement Mypy.

mypy – Optional static typing for Python