PEP 746: TypedMetadata for type checking of PEP 593 Annotated

[mikeshardmind (Michael H)]

@adriangb

Would you be willing to back a concrete proposal for refinement types as you see them?

Yes, including contributing time on specification language if that’s needed.

@adriangb

I want to make sure we’re not choosing to hold out on improving an existing useful pattern that many, many Python users are using in favor of a possible future feature that has no strong backing.

Very understandable. As far as I see it, I have no strong opposition to your second point here, and the first one is only in ensuring that we specify it in a way where it composes properly, not to change the actual thing being enabled.

@adriangb

Can’t we make progress with what we have right now and then augment it with better support in the type system later if they are not mutually incompatible?

Usually, I’d agree, but in a foreseeable case like this one, I’d rather not out of consideration for both the people who will need to implement support for then both of these into type checkers, as well as potential churn for library authors and devs.

As for foreseeable incompatibility:

@davidfstr

FWIW, the proposed handling of TypeForm in combination with Annotated[...] is already defined to work that way, stripping out the metadata component:

count: int | str = -1
if ismatch(count, Annotated[int, ValueRange(1, float('inf'))]):
    assert_type(count, int)  # NOT: Annotated[int, ...]
else:
    assert_type(count, int | str)

There’s another pep being drafted and revised as we discuss this one that would interact poorly