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python
PEP 661 -- Sentinel Values
Please consider PEP 661 -- Sentinel Values https://peps.python.org/pep-0661/
- [X] The PEP has been discussed in threads listed in its
Post-Historyheader - [X] The PEP was announced on Discuss (link in
Post-History) - [X] The PEP includes all relevant Suggested Sections
- [X] The PEP includes endorsements from the projects/groups/people it helps
- [X] The PEP has a CODEOWNERS entry
Since this PEP proposes a change to the type system (an extension of the values allowed in Literal[]), the Typing Council is also looking at it. We'll discuss internally and share our opinion here if the Steering Council thinks it's helpful.
Note that there is some more discussion on Discourse, and the PEP might need to be changed.
On behalf of the Typing Council:
The PEP currently proposes to use Literal[SentinelName] as the type annotation to describe the type of a sentinel object. We prefer to support SentinelName directly without the need to wrap it in a Literal . This would require the sentinel base class to supply a __or__ method to support unions, which would be straightforward. The most common sentinel value used in Python is None, and it is special-cased in the type system to not require Literal[None] in a type annotation. Since type checkers (both static and runtime) will already need to special-case sentinels, we wouldn’t gain any simplicity by forcing users to wrap sentinels in Literal[].
A suggestion came up to allow both forms in type annotations, Literal[SentinelName] and SentinelName. We prefer to keep only the latter so we don’t introduce redundant ways to describe the same type.
(Note from me personally: The change we're recommending here aligns with something Tal already said he was planning to change in the PEP, leading to Petr's message above.)
Thank you @JelleZijlstra and members of the Typing Council! Your opinions and feedback are greatly appreciated.
I now agree that SentinelName is preferable to Literal[SentinelName]. I greatly prefer only SentinelName rather than both options.
Please note that the PEP has been amended to reflect the changes to typing as described by @JelleZijlstra above.
FYI, the current SC decided to leave this PEP for the next SC to decide, given that SC elections are very soon (https://discuss.python.org/t/steering-council-nominations-will-open-november-7th-2024).
I've created a PR with suggested post-submission changes to the PEP proposal. https://github.com/python/peps/pull/4232
I understand that the SC has recently begun reviewing this proposal. If possible, please consider the proposal to include these changes.
Also, please note that the current proposal suggests adding a new stdlib module, "sentinellib". It has been suggested to add a new "sentinel" builtin, instead, or to add this to the "typing" module; I consider all of these to be valid options.
SC review status: still on our radar, we're catching up on the latest in the Discuss thread before we can make any decisions.
The SC is waiting for a response on our offer of office hours to @taleinat
Until then we are in a holding pattern for PEP 661.
