Use PEP 661 (sentinels) in the standard library

[JelleZijlstra]

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

[JelleZijlstra]

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.