gh-148829: Implement PEP 661 by JelleZijlstra · Pull Request #148831 · python/cpython · GitHub

Replaces JelleZijlstra#4. See python/steering-council#258.

Issue: Implement PEP 661 Sentinel Values #148829

📚 Documentation preview 📚: https://cpython-previews--148831.org.readthedocs.build/

Co-authored-by: Victorien <65306057+Viicos@users.noreply.github.com>

Co-authored-by: Pieter Eendebak <pieter.eendebak@gmail.com>

🤖 New build scheduled with the buildbot fleet by @JelleZijlstra for commit 53a84b6 🤖

Results will be shown at:

https://buildbot.python.org/all/#/grid?branch=refs%2Fpull%2F148831%2Fmerge

If you want to schedule another build, you need to add the 🔨 test-with-buildbots label again.

The C code and the corresponding documentation look good to me. I didn't look very closely at the tests.

LGTM

Clarify the pickle document. So that people will see X = sentinel("Y") is not picklable.

Buildbot run: A few failures look unrelated

In file included from ./Include/internal/pycore_backoff.h:12,
                 from ./Include/internal/pycore_code.h:11,
                 from ./Include/internal/pycore_call.h:11,
                 from ./Modules/_ctypes/_ctypes.c:109:
./Modules/_ctypes/_ctypes.c: In function ‘set_stginfo_ffi_type_pointer’:
./Modules/_ctypes/_ctypes.c:2234:40: error: ‘FFI_TYPE_COMPLEX’ undeclared (first use in this function); did you mean ‘FFI_TYPE_DOUBLE’?
         assert(fmt->pffi_type->type == FFI_TYPE_COMPLEX);
                                        ^~~~~~~~~~~~~~~~
./Modules/_ctypes/_ctypes.c:2234:40: note: each undeclared identifier is reported only once for each function it appears in

But I also see

Traceback (most recent call last):
  File "/home/ubuntu/buildarea/pull_request.stan-aarch64-ubuntu.oddballs/build/Lib/test/pickletester.py", line 3254, in test_builtin_types
    s = self.dumps(t, proto)
  File "/home/ubuntu/buildarea/pull_request.stan-aarch64-ubuntu.oddballs/build/Lib/test/test_xpickle.py", line 190, in dumps
    return self.send_to_worker(python, data)
           ~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^
  File "/home/ubuntu/buildarea/pull_request.stan-aarch64-ubuntu.oddballs/build/Lib/test/test_xpickle.py", line 172, in send_to_worker
    raise exception
AttributeError: Can't get attribute 'sentinel' on <module 'builtins' (built-in)>

https://buildbot.python.org/#/builders/1865/builds/54

Which is a bit mysterious, is this looking at the builtins of another build?

Which is a bit mysterious, is this looking at the builtins of another build?

Yes. We need to patch this test. It tests pickle compatiblity acrosses python versions.

./python -m test test_xpickle -m test_builtin_types -u xpickle

diff --git a/Lib/test/pickletester.py b/Lib/test/pickletester.py
index 6366f12257..c2018c9785 100644
--- a/Lib/test/pickletester.py
+++ b/Lib/test/pickletester.py
@@ -3244,6 +3244,7 @@ def test_builtin_types(self):
             'BuiltinImporter': (3, 3),
             'str': (3, 4),  # not interoperable with Python < 3.4
             'frozendict': (3, 15),
+            'sentinel': (3, 15),
         }
         for t in builtins.__dict__.values():
             if isinstance(t, type) and not issubclass(t, BaseException):

Buildbot run: A few failures look unrelated

I opened #148967 to track it.

Thanks for the feedback! Pushed a new version.

🤖 New build scheduled with the buildbot fleet by @JelleZijlstra for commit 7c276c5 🤖

Results will be shown at:

https://buildbot.python.org/all/#/grid?branch=refs%2Fpull%2F148831%2Fmerge

If you want to schedule another build, you need to add the 🔨 test-with-buildbots label again.

Buildbot failures seem to be just the FFI_TYPE_COMPLEX thing on RHEL8.

The Include/sentinelobject.h header file should be moved to Include/cpython/ since its C API is excluded from the limited C API. I created PR gh-149186 to move the header file.

JelleZijlstra and others added 11 commits April 16, 2026 20:45

Initial implementation of PEP 661 v2 (sentinels)

559e527

Update Doc/library/functions.rst

ededfb7

Co-authored-by: Victorien <65306057+Viicos@users.noreply.github.com>

Update Doc/library/functions.rst

39f364c

Co-authored-by: Victorien <65306057+Viicos@users.noreply.github.com>

gc, name

0adf314

C API

e4a106a

not needed

e5b8789

use it for NoExtraItems

bdcb400

tweaks

cc4545c

fix warning?

be58215

blurb

91de05b

attribute

4a970a6

JelleZijlstra requested review from a team, AA-Turner, AlexWaygood, ZeroIntensity, emmatyping, ericsnowcurrently and erlend-aasland as code owners April 21, 2026 13:59

bedevere-app Bot added the awaiting core review label Apr 21, 2026

bedevere-app Bot mentioned this pull request Apr 21, 2026

Implement PEP 661 Sentinel Values #148829

Closed

JelleZijlstra added the DO-NOT-MERGE label Apr 21, 2026

JelleZijlstra mentioned this pull request Apr 21, 2026

PEP 661 -- Sentinel Values python/steering-council#258

Closed

5 tasks done

johnslavik self-requested a review April 22, 2026 09:44

eendebakpt reviewed Apr 22, 2026

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eendebakpt reviewed Apr 22, 2026

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eendebakpt reviewed Apr 22, 2026

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Comment thread Objects/sentinelobject.c

refactors

9cfd101

eendebakpt reviewed Apr 22, 2026

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Comment thread Objects/sentinelobject.c Outdated

Update Objects/sentinelobject.c

a851235

Co-authored-by: Pieter Eendebak <pieter.eendebak@gmail.com>

JelleZijlstra added the 🔨 test-with-buildbots Test PR w/ buildbots; report in status section label Apr 24, 2026

bedevere-bot removed the 🔨 test-with-buildbots Test PR w/ buildbots; report in status section label Apr 24, 2026

ZeroIntensity approved these changes Apr 24, 2026

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bedevere-app Bot added awaiting merge and removed awaiting core review labels Apr 24, 2026

sunmy2019 reviewed Apr 24, 2026

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Comment thread Objects/sentinelobject.c

sunmy2019 approved these changes Apr 24, 2026

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sunmy2019 reviewed Apr 24, 2026

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Comment thread Doc/c-api/sentinel.rst Outdated

Comment thread Doc/library/functions.rst Outdated

picnixz reviewed Apr 24, 2026

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JelleZijlstra added 2 commits April 24, 2026 19:27

Merge remote-tracking branch 'upstream/main' into pep661

396356e

feedback

bfa98a1

JelleZijlstra added 2 commits April 24, 2026 23:00

add assert

465061a

another one

70c04a2

picnixz reviewed Apr 25, 2026

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Comment thread Lib/test/test_capi/test_object.py Outdated

Comment thread Doc/library/functions.rst

Comment thread Lib/test/test_builtin.py

Comment thread Lib/test/test_builtin.py

sunmy2019 reviewed Apr 25, 2026

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Comment thread Doc/library/functions.rst Outdated

more tweaks

7c276c5

JelleZijlstra added the 🔨 test-with-buildbots Test PR w/ buildbots; report in status section label Apr 27, 2026

bedevere-bot removed the 🔨 test-with-buildbots Test PR w/ buildbots; report in status section label Apr 27, 2026

JelleZijlstra merged commit 29a92ab into python:main Apr 28, 2026
107 of 117 checks passed

bedevere-app Bot removed the awaiting merge label Apr 28, 2026

NekoAsakura mentioned this pull request Apr 29, 2026

gh-148874: add flag to CALL to skip periodic check in with #149019

Closed

gh-148829: Implement PEP 661 by JelleZijlstra · Pull Request #148831 · python/cpython · GitHub

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gh-148829: Implement PEP 661 - #148831

gh-148829: Implement PEP 661#148831JelleZijlstra merged 21 commits intopython:mainpython/cpython:mainfrom JelleZijlstra:pep661JelleZijlstra/cpython:pep661Copy head branch name to clipboard

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gh-148829: Implement PEP 661#148831
JelleZijlstra merged 21 commits into
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