x = []
x.append(1)
x[0] = "new"
x[0] + "oops"
const x = []
x.push(1)
type t = typeof x
// ^? type t = number[]
x[0] = "new"
type t2 = typeof x
// ^? type t2 = (number | string)[]
const y = x[0] + "oops"
// ^? const y: string
Either way, you didn't annotate the code so it's kind of pointless to discuss.
Also fwiw python is typed regardless of the annotations; types are not optional in any sense. Unless you're using BCPL or forth or something like that
There are several literals in that code snippet; I could annotate them with their types, and this code would still be exactly as it is. You asked why there are competing type checkers, and the fact that the language is only optionally typed means ambiguity like that example exists, and should be a warning/bug/allowed; choose the type checker that most closely matches the semantics you want to impose.
So only the outer API surface of the library matters. That’s mostly explicitly typed functions and classes so the room for different interpretations is lower (but not gone).
This is obviously out the window for libraries like Pydantic, Django etc with type semantics that aren’t really covered by the spec.
I think everyone basically agrees that at the package boundary, you want explicit types, but inside application code things are much more murky.
(plus of course, performance, particularly around incremental processing, which Astral is specifically calling out as a design goal here)
Yes, but in practice, the ecosystem mostly tests against mypy. pyright has been making some inroads, mostly because it backs the diagnostics of the default VS Code Python extension.
> Do application developers need to limit their use of libraries to ones that support their particular choice of type checker?
You can provide your own type stubs instead of using the library's built-in types or existing stubs.
Do library authors have to test against every type checker to ensure maximum compatibility? Do application developers need to limit their use of libraries to ones that support their particular choice of type checker?