Python Type Checker Comparison: Empty Container Inference

[Boxxed]

My favorite part about the type annotations in python is that it steers you into a sane subset of the language. I feel like it's kind of telling that python is this super dynamic language but the type annotations aren't powerful enough to denote all that craziness.

[jez]

A more complicated version of this problem exists in TypeScript and Ruby, where there are only arrays. Python’s case is considerably simpler by also having tuples, whose length is fixed at the time of assignment.

In Python, `x = []` should always have a `list[…]` type inferred. In TypeScript and Ruby, the inferred type needs to account for the fact that `x` is valid to pass to a function which takes the empty tuple (empty array literal type) as well as a function that takes an array. So the Python strategy #1 in the article of defaulting to `list[Any]` does not work because it rejects passing `[]` to a function declared as taking `[]`.

[loevborg]

FWIW, Typescript is using Strategy 2: https://www.typescriptlang.org/play/?#code/GYVwdgxgLglg9mABM...

I'm a bit confused by the fact that the array starts out typed as `any[]` (e.g. if you hover over the declaration) but then, later on, the type gets refined to `(string | number)[]`. IMO it would be nicer if the declaration already showed the inferred type on hover.

[sheept]

I agree, it's always been unsettling to see any[] on hover, even though it gets typed in the end.

I think one reason might be to allow the type to be refined differently in different code paths. For example:

    function x () {
        let arr = []
        if (Math.random() < 0.5) {
            arr.push(0)
            return arr
        } else {
            arr.push('0')
            return arr
        }
    }

In each branch, arr is typed as number[] and string[], respectively, and x's return type is number[] | string[]. If it decided to retroactively infer the type of arr at declaration, then I'd imagine x's return type would be the less specific (number | string)[].

[bastawhiz]

It depends on your tsconfig. An empty array could be typed as never[], forcing you to annotate it.

[IshKebab]

I think it would be worth mentioning that in normal use (strict mode) Pyright simply requires you to add type annotations to the declaration. Occasionally mildly annoying but IMO it's clearly the best option.

[curiousgal]

I can't help but find type hints in python to be..goofy? I have a colleague who has a substantial C++ background and now working in python, the code is just littered with TypeAlias, Generic, cast, long Unions etc.. this can't be the way..

[tialaramex]

Typing is a relatively easy way for the human author and the machine to notice if they disagree about what's going on before problems arise. It is unfortunate that Python doesn't do a good job with types, I was reading earlier today about the mess they made of booleans - their bool type is actually just the integers again.

[wiseowise]

What is the way in your opinion?

[IshKebab]

I strongly disagree. Python has actually done a decent job of adding type annotations into the language IMO.

If you ignore the bit where they don't actually specify their semantics anyway.

> this can't be the way..

The alternative is fragile and unmaintainable code. I know which I prefer!