PEP 746: TypedMetadata for type checking of PEP 593 Annotated

[mikeshardmind (Michael H)]

@adriangb

I don’t think this is true.

This PEP adds an Annotated type to the typing module to decorate existing types with context-specific metadata. Specifically, a type T can be annotated with metadata x via the typehintAnnotated[T, x] . This metadata can be used for either static analysis or at runtime

The entire motivation section of that pep then goes on to point exclusively at cases outside the type system and using this as a surrogate for things yet to be included in the type system, and does not suggest that type checkers would be the consumer of the annotations, and even directly states (including what has survived to the living specification) that tools may choose what annotations they consume, this seems to preclude mandating that this be part of the type system to me when viewed with proper context.

@adriangb

It seems that you agree with this but don’t think it’s worth taking the step of making the existing and widely used Annotated[int, Gt(0)] type checkable (even if the refinement isn’t enforced) and instead we should be introducing refinement types, along with new syntax, into the language? Keep in mind that my proposal can be back ported to Python 3.8, refinement types wouldn’t benefit users for years to come. And they aren’t mutually exclusive.

I don’t think there’s anything preventing a backport via something as simple as:

typing_extensions.Refinement[int, "< 5"]

this might not be the ideal expression of this, but considering that I imagine refinement types would be limited in what they allow for valid constraints, not allowing arbitrary constraints, it’s certainly not the worst.

Maybe that’s even just a better form in general than the specific new syntax, but I don’t really like it being “stringly-typed”