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gh-148829: Add PySentinel_CheckExact()#149725

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scoder:pysentinel_checkexact
May 13, 2026
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gh-148829: Add PySentinel_CheckExact()#149725
vstinner merged 5 commits into
python:mainfrom
scoder:pysentinel_checkexact

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@scoder scoder commented May 12, 2026 •
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As suggested in https://discuss.python.org/t/pep-661-sentinel-values/9126/343, this PR adds a PySentinel_CheckExact() C macro and makes PySentinel_Check() an alias, which can change if/when we allow subtypes in the future.

The discussion seems to suggest that subtyping will be valuable at some point. Adding both C macros now allows to write concise code on user side that (hopefully) requires less changes later.

I chose to make PySentinel_Check() an alias of PySentinel_CheckExact() and not the other way round because the current C macro actually does an exact check, so its correct name is PySentinel_CheckExact().

@scoder scoder requested a review from ZeroIntensity as a code owner May 12, 2026 14:52
@scoder scoder added the needs backport to 3.15 pre-release feature fixes, bugs and security fixes label May 12, 2026
@bedevere-app bedevere-app Bot mentioned this pull request May 12, 2026
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read-the-docs-community Bot commented May 12, 2026 •
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Documentation build overview

📚 cpython-previews | 🛠️ Build #32670091 | 📁 Comparing 03f28f7 against main (0534774)

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20 files changed · ± 20 modified

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scoder commented May 12, 2026

I set the skipNews tag since we're still pre-release, but OTOH, we're post-beta1, so we might still want to add a news entry. Should I add one?

I was also mildly surprised to find the tests in object.c. By name, that seems quite generic for tests that belong to a specific builtin type.

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scoder commented May 12, 2026

@vstinner @JelleZijlstra

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JelleZijlstra commented May 12, 2026

Ah, sorry I forgot to follow up on this. I'm OK with the change and we should be able to do it in the beta phase, but we will need a NEWS entry since we already made a (beta) release with the old logic.

@scoder scoder removed the skip news label May 12, 2026
StanFromIreland
StanFromIreland reviewed May 12, 2026
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Comment thread Misc/NEWS.d/next/C_API/2026-05-12-16-47-21.gh-issue-149725.HZLBTZ.rst Outdated
Comment thread Doc/c-api/sentinel.rst Outdated
Comment thread Doc/c-api/sentinel.rst Outdated
Comment thread Doc/c-api/sentinel.rst Outdated
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vstinner approved these changes May 13, 2026
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LGTM

Comment thread Doc/c-api/sentinel.rst
Return true if *o* is a :class:`sentinel` object or a subtype.
The :class:`sentinel` type does not currently allow subclasses,
so this check is exact.
Future Python versions may choose to allow subtyping.
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Can you add "This function always succeeds." to both functions?

@vstinner vstinner enabled auto-merge (squash) May 13, 2026 09:44
@vstinner vstinner merged commit 94df625 into python:main May 13, 2026
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miss-islington-app Bot commented May 13, 2026

Thanks @scoder for the PR, and @vstinner for merging it 🌮🎉.. I'm working now to backport this PR to: 3.15.
🐍🍒⛏🤖

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bedevere-app Bot commented May 13, 2026

GH-149766 is a backport of this pull request to the 3.15 branch.

@bedevere-app bedevere-app Bot removed the needs backport to 3.15 pre-release feature fixes, bugs and security fixes label May 13, 2026
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vstinner commented May 13, 2026

Merged. Thanks @scoder for your contribution!

vstinner pushed a commit that referenced this pull request May 13, 2026
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[3.15] gh-148829: Add PySentinel_CheckExact() (GH-149725) (#149766)
a5f77a1 
gh-148829: Add PySentinel_CheckExact() (GH-149725)
(cherry picked from commit 94df625)

Co-authored-by: scoder <stefan_ml@behnel.de>
@scoder scoder deleted the pysentinel_checkexact branch May 13, 2026 12:16
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