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Comment by i80and | Hacker News Reader

> better type checking of C (over Python).

Note that Python's official type checker (mypy) is pretty good (within the limits of type erasure); I started the project I've been leading up at work this past year in statically-typed Python, and it's been a great experience.

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I tried to like mypy, really did. But it always ended up in some kind of circular import tailspin for me. C++ gets around the problem by allowing separation of interface and implementation, Haskell by allowing circular imports. Maybe I missed something, maybe it's fixed by now, but that was my experience a couple of years ago.

They made a flag TYPE_CHECKING to avoid circular import in type checking. Its ugly but works.

https://docs.python.org/3/library/typing.html#typing.TYPE_CH...

Also the circular imports was the least of the problems I’ve run into with mypy. They still can’t model a JSON type because it is recursive.

Recursive types were added in Typescript 3.7. Hopefully Python takes a page from their book.

Can you show example? I believe you can use TypedDict to express that.

It’s roughly

    JSON = Union[Dict[str, JSON], List[JSON], str, bool, int, NoneType]

TypedDict isn’t for arbitrary JSON, but it is really cool.

Ah I see, TypedDict won't be useful here.

Looks like there is a work in progress though[1] and people in the ticket provided some workarounds[2][3] for now.

[1] https://github.com/python/mypy/issues/731

[2] https://github.com/python/mypy/issues/731#issuecomment-53990...

[3] https://gist.github.com/catb0t/bd82f7815b7e95b5dd3c3ad294f3c...

I've played with mypy a little, and it's certainly not bad, but it does suffer by comparison to typescript, which just has a lot of really great structural typing features that mypy still lacks.

mypy recently got structural typing. They call it Protocols, which isn't the first thing one one put into google when looking for this feature. It's not as powerful and terse as typescript unfortunately.

They also added TypedDict recently https://mypy.readthedocs.io/en/latest/more_types.html

Yeah, I didn't mean to imply no features, just far less than TS.

I haven't tried mypy yet but want to get to it. It sounds very interesting! After using Python so much the last several years, I've gotten more curmudgeonly about strong type checking in my programming languages.

Don't wait for a new project, just start using it (is most beneficial if for every function you specify types for parameters and output). If you use an IDE that understand it like PyCharm, suddenly the autocomplete will start working right, so will refactoring and it will start highlighting potential errors.

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