Description

JelleZijlstra

opened on Apr 28, 2026

Issue body actions

Feature or enhancement

Proposal:

I just merged support for PEP-661, which adds the new sentinel() builtin. Let's use it in the standard library so we can simplify some code and stress-test the feature.

We'll have to be careful to preserve compatibility with features such as repr() and boolean values.

Has this already been discussed elsewhere?

No response given

Links to previous discussion of this feature:

No response

Linked PRs

• gh-149083: Convert some private and simple sentinels to PEP 661 #149084
• gh-149083: Convert dataclasses.MISSING and dataclasses.KW_ONLY to sentinels #149086
• gh-149083: Make functools.Placeholder a sentinel #149088
• gh-149083: Use sentinel in functools.rst docs #149176
• gh-149083: Change several other docs examples to use sentinel() #149213
• gh-149083: Convert _initial_missing for pure py reduce to sentinel #149536
• gh-149083: use sentinel to fix _functools.reduce() signature #149591
• [3.15] gh-149083: Convert _initial_missing for pure py reduce to sentinel (GH-149536) #149592
• gh-149083: Use sentinel for urllib.parse._UNSPECIFIED #149612
• gh-149083: Use sentinel for os.path.{ALLOW_MISSING,ALL_BUT_LAST} #149613
• [3.15] gh-149083: use sentinel to fix _functools.reduce() signature (GH-149591) #149653
• [3.15] gh-149083: Use sentinel for urllib.parse._UNSPECIFIED (GH-149612) #151017
• gh-149083: Document that dataclasses.MISSING and KW_ONLY are sentinels #155919
• [3.15] gh-149083: Document that dataclasses.MISSING and KW_ONLY are sentinels (GH-155919) #155927

Activity

1. JelleZijlstra

   added

   type-featureA feature request or enhancementA feature request or enhancement

   on Apr 28, 2026
2. JelleZijlstra

   self-assigned this

   on Apr 28, 2026
3. JelleZijlstra commented on Apr 28, 2026

   

   JelleZijlstra

   on Apr 28, 2026

   MemberAuthor

   More actions

   I have a local patch converting typing.NoDefault to a sentinel: 1ce5c13

   However, it breaks two tests in test_typing:

   ======================================================================
   ERROR: test_constructor (test.test_typing.NoDefaultTests.test_constructor)
   ----------------------------------------------------------------------
   Traceback (most recent call last):
     File "/Users/jelle/py/cpython/Lib/test/test_typing.py", line 11104, in test_constructor
       self.assertIs(NoDefault, type(NoDefault)())
                                ~~~~~~~~~~~~~~~^^
   TypeError: sentinel expected 1 argument, got 0
   
   ======================================================================
   FAIL: test_repr (test.test_typing.NoDefaultTests.test_repr)
   ----------------------------------------------------------------------
   Traceback (most recent call last):
     File "/Users/jelle/py/cpython/Lib/test/test_typing.py", line 11109, in test_repr
       self.assertEqual(repr(NoDefault), 'typing.NoDefault')
       ~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   AssertionError: 'NoDefault' != 'typing.NoDefault'
   - NoDefault
   + typing.NoDefault

   We can perhaps live with those behavior changes but I think I'll leave this one be.
4. StanFromIreland

   added

   stdlibStandard Library Python modules in the Lib/ directoryStandard Library Python modules in the Lib/ directory

   type-refactorCode refactoring (with no changes in behavior)Code refactoring (with no changes in behavior)

   and removed

   type-featureA feature request or enhancementA feature request or enhancement

   on Apr 28, 2026
5. JelleZijlstra

   added a commit that references this issue on Apr 30, 2026

   gh-149083: Convert dataclasses.MISSING and dataclasses.KW_ONLY to sen…

   1695221
6. sobolevn

   added 3 commits that reference this issue on Apr 30, 2026

   gh-149083: Use `sentinel` in `functools.rst` docs (#149176)

   bf42481

   pythongh-149083: Change several other docs examples to use `sentinel()`

   9dd3492

   gh-149083: Change several other docs examples to use `sentinel()` (#1…

   690e0de
7. sobolevn

   added 2 commits that reference this issue on May 9, 2026

   gh-149083: Convert `_initial_missing` for pure py `reduce` to `sentin…

   bc8cf07

   [3.15] gh-149083: Convert `_initial_missing` for pure py `reduce` to …

   915b6c1
8. JelleZijlstra

   added a commit that references this issue on May 9, 2026

   pythongh-149083: Use sentinel for os.path.{ALLOW_MISSING,ALL_BUT_LAST}

   20b62f7
9. serhiy-storchaka commented on May 9, 2026

   

   serhiy-storchaka

   on May 9, 2026

   Member

   More actions

   There are two, or even three, types of sentinels.

   • Intended to be used by the user, like dataclasses.KW_ONLY or os.path.ALL_BUT_LAST. Their repr should refer to the name by which they are accessible. Pickleability is also a good bonus.
   • Intended to be used as placeholders for default argument values. They should not be accessible by user. Their repr can be <not specified> or something like. They are not intended to be pickled either.
   • For internal use. The same as the previous case, but usually users should not see them at all.
10. JelleZijlstra

    added 2 commits that reference this issue on May 10, 2026

    gh-149083: use sentinel to fix _functools.reduce() signature (#149591)

    c6fd7de

    [3.15] gh-149083: use sentinel to fix _functools.reduce() signature (G…

    73c80cb
11. JelleZijlstra

    added 2 commits that reference this issue on Jun 6, 2026

    gh-149083: Use sentinel for urllib.parse._UNSPECIFIED (#149612)

    884ac3e

    [3.15] gh-149083: Use sentinel for urllib.parse._UNSPECIFIED (GH-149612…

    9352936
12. philthompson10

    added a commit that references this issue on Jun 17, 2026

    pythongh-149083: Use sentinel for urllib.parse._UNSPECIFIED (python#1…

    e4308c5
13. hugovk

    added 2 commits that reference this issue on Aug 17, 2026

    gh-149083: Document that dataclasses.MISSING and KW_ONLY are sentinels (

    e13c3d6

    [3.15] gh-149083: Document that dataclasses.MISSING and KW_ONLY are s…

    ac4e5d2
14. picnixz

    added a commit that references this issue on Nov 1, 2026

    gh-149083: Make functools.Placeholder a sentinel (#149088)

    ee074d1

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