Odd error message for invalid use of tuple(): expected "Iterable[Never]"

[wyattscarpenter]

tuple(1) is invalid, of course. But what error message does it bring?

main.py:1: error: Argument 1 to "tuple" has incompatible type "int"; expected "Iterable[Never]"  [arg-type]

Well that's odd! Considering that it can take iterables of all sorts of things, and not... impossible things.

In fact, this has to be wrong, because when I make my own function with Iterable[Never] argument, it doesn't like taking iterables of other things

from typing import Iterable, Never

def f(a: Iterable[Never]) -> None:
    pass

f([1,2,3]) # main.py:6: error: Argument 1 to "f" has incompatible type "list[int]"; expected "Iterable[Never]"  [arg-type]

[sterliakov]

Tuple constructor takes Iterable[T] where T is a type variable. Mypy cannot infer any value of T such that int is a subtype of Iterable[T] and so has to say Never - that's what we do with all unsolvable type variables. I agree this is questionable from usability standpoint.

[wyattscarpenter]

Ah, yes, it's the same for any user-level function as well...

from typing import Iterable, Never

def f[T](a: Iterable[T]) -> None:
    pass

f(1) # main.py:6: error: Argument 1 to "f" has incompatible type "int"; expected "Iterable[Never]"  [arg-type]

[wyattscarpenter]

It's hard to say what the type checker could usefully and accurately say here. Never is too narrow, Any is too broad, int (the actual value of T in this case) is also unhelpful). I guess the best option might be just printing the name of the typevar instead.

[sterliakov]

But typevar name is also weird, some people use long-ish typevar names, and that can look like some "good" type one has to look up. Even short names like T won't really friendly for beginners who aren't familiar with typevars (or at least not proficient with typing to expect them in such contexts).

[A5rocks]

We used to use Unknown <nothing> in the error message IIRC and maybe that's cleaner? Alternatively maybe we can also emit a "unable to infer type vars" just like with variable assignment if a function call fails?

(I think any sort of proper solution for this is really hard, I would love an interface to explore a program and find what constraints are used for inference etc, but that's a massive undertaking...)

[wyattscarpenter]

Maybe instead of <nothing>, it should print <something>. Probably kind of hard to get it to know the difference between Nevers that are something not given and Nevers that are real Nevers, though...

[A5rocks]

It's very easy, the Never is explicitly assigned upon not finding anything to infer. We would just need to add some sort of "kind" parameter like Any I think.