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PEP 661 sentinel support #3589
Description
Activity
Thanks for reporting. I'm not clear on the status of sentinel support in Pyrefly ATM, so would need to look into that as a first step
- changed the title
[-]Type aliases of PEP661 sentinels say "Expected ... to be a type alias"[/-][+]PEP 661 sentinel support[/+]on May 27, 2026 I'm pretty sure we just don't support it yet because it was just accepted
This is now solved on the latest main branch with the merging of #3575
The only edge-case I could see with the testing of this snippet is that you have to use the sentinel itself, AUTO, in narrowing operations like
is AUTO/is not AUTO, you can't use the type alias. So something like this doesn't work:from typing import TypeAlias from typing_extensions import Sentinel AUTO = Sentinel("AUTO") ToLoopyTypeConvertible: TypeAlias = AUTO def foo(a: int | ToLoopyTypeConvertible) -> None: if a is ToLoopyTypeConvertible: reveal_type(a)
But I'm not sure if you are supposed to be able to use the type alias for narrowing, I don't think PEP 661 mentioned anything regarding that, so this is probably fine. If not I can take a look at adding support for that as well.
Reacted by Rebecca ChenYeah, I think we can close this issue now. I checked, and pyright allows
ToLoopyTypeConvertibleto be used in place ofAUTOin the type annotation but not for narrowing, so pyrefly's behavior matches pyright there.Sentinel support will be available in pyrefly version 1.1.0.
Describe the Bug
When running pyrefly on this snippet:
configured with
preset = "strict", I get the following error:basedpyright (and pyright configured with experimental features) accepts this code.
This is with pyrefly 1.0.0. The sandbox link reproduces the issue for me.
Sandbox Link
https://pyrefly.org/sandbox/?project=N4IgZglgNgpgziAXKOBDAdgEwEYHsAeAdAA4CeS4ATrgLYAEA%2Bg2AK4AuLlMTdENxuSmzoZ0uNqjYRc6OAB10CsNXptSxCOgDmvfoOEAVdTACCUCKnmL0y2nTUbtDGPjYxZ02boFC6AZXcpdBgoBTD0EwBVAwB5OgBef0DNEIAKORAo2IyASnCFA1wAGVxcMiNiGABhGQA3GCEIbFhEOgrTc0sEuiyYhRAAGhAyLjAoUkI2WigKYi44GGFEjLg2SggAYzYMkABfIdQtiHqAMWgYCjQsPCIyPaA
(Only applicable for extension issues) IDE Information
No response