[notatallshaw]

I assume in your example if you update the foo declaration to the following it solves the complaint:

    foo: list[str] = []

If so this a type checking design choice:

  * What can I infer from an empty collection declaration?
  * What do I allow further down the code to update inferences further up the code?

I don't know Pyrefly's philosophy here, but I assume it's guided by opinionated coding guidelines inside Meta, not what is perhaps the easiest for users to understand.

[eqvinox]

Yes, annotating the type explicitly fixes it; but tbh I'd consider that type annotation "unnecessary/distracting code litter".

As far as their philosophy goes, it's an open issue they're working on, so their philosophy seems to agree this particular pattern should work :)

[sh34r]

It is a purely subjective design decision, but I personally prefer the stricter rules that don’t do backwards type inferences like this… type hints shouldn’t follow duck typing semantics. Otherwise you’re not providing nearly as much value IMO. Typescript is really the model organism here. They took the most cursed mainstream programming language, and made it downright good.

Today, the “: list[str]” is 11 wasted characters and it’s not as aesthetically pleasing. Tomorrow, you do some refactor and your inferred list[str] becomes a list[int] without you realizing it… I’m sure that sounds silly in this toy example, but just imagine it’s a much more complex piece of code. The fact of the matter is, you declared foo as a list[any] and you’re passing it to a function that takes an iterable[str] — it ought to complain at you! Type hints are optional in Python, and you can tell linters to suppress warnings in a scope with a comment too.

That being said, perhaps these more permissive rules might be useful in a legacy codebase where no type annotations exist yet.

Really, it’d be extra nice if they made this behavior configurable, but I’m probably asking for too much there. What’s next, a transpiler? Dogs and cats living together?!

[notatallshaw]

I agree, but as a type checker it is a subjective choice, whether to be explicit and not make assumptions or whether to infer from certain certain patterns as probably correct and not be noisy to the user. Very glad to see they plan to fix this.