From 16952218d0535904236e8a39851133688c9ce1f0 Mon Sep 17 00:00:00 2001
From: Jelle Zijlstra <jelle.zijlstra@gmail.com>
Date: Wed, 29 Apr 2026 19:39:29 -0700
Subject: [PATCH] gh-149083: Convert dataclasses.MISSING and
 dataclasses.KW_ONLY to sentinels (#149086)

There were comments claiming these were implemented as custom classes to give a nicer
repr(), but the repr() wasn't all that nice:

>>> repr(dataclasses.MISSING)
'<dataclasses._MISSING_TYPE object at 0x1005e7e00>'
>>> repr(dataclasses.KW_ONLY)
'<dataclasses._KW_ONLY_TYPE object at 0x100884050>'

Sentinels are conceptually the right tool for these, so let's use them.

This does change the repr() of these two objects.
---
 Lib/dataclasses.py                                  | 13 ++++---------
 Lib/test/test_dataclasses/__init__.py               |  2 +-
 .../2026-04-27-20-15-54.gh-issue-149083.BdrpU8.rst  |  2 ++
 3 files changed, 7 insertions(+), 10 deletions(-)
 create mode 100644 Misc/NEWS.d/next/Library/2026-04-27-20-15-54.gh-issue-149083.BdrpU8.rst

diff --git a/Lib/dataclasses.py b/Lib/dataclasses.py
index df192763c5bd15..e9810d6bd5d57b 100644
--- a/Lib/dataclasses.py
+++ b/Lib/dataclasses.py
@@ -178,17 +178,12 @@ def __repr__(self):
         return '<factory>'
 _HAS_DEFAULT_FACTORY = _HAS_DEFAULT_FACTORY_CLASS()
 
-# A sentinel object to detect if a parameter is supplied or not.  Use
-# a class to give it a better repr.
-class _MISSING_TYPE:
-    pass
-MISSING = _MISSING_TYPE()
+# A sentinel object to detect if a parameter is supplied or not.
+MISSING = sentinel("MISSING")
 
 # A sentinel object to indicate that following fields are keyword-only by
-# default.  Use a class to give it a better repr.
-class _KW_ONLY_TYPE:
-    pass
-KW_ONLY = _KW_ONLY_TYPE()
+# default.
+KW_ONLY = sentinel("KW_ONLY")
 
 # Since most per-field metadata will be unused, create an empty
 # read-only dictionary that can be shared among all fields.
diff --git a/Lib/test/test_dataclasses/__init__.py b/Lib/test/test_dataclasses/__init__.py
index 5eec9e23cd414b..8a0a7d12c04aa4 100644
--- a/Lib/test/test_dataclasses/__init__.py
+++ b/Lib/test/test_dataclasses/__init__.py
@@ -995,7 +995,7 @@ class D:
         self.assertNotIn('x', D.__dict__)
 
     def test_missing_repr(self):
-        self.assertIn('MISSING_TYPE object', repr(MISSING))
+        self.assertEqual(repr(MISSING), 'MISSING')
 
     def test_dont_include_other_annotations(self):
         @dataclass
diff --git a/Misc/NEWS.d/next/Library/2026-04-27-20-15-54.gh-issue-149083.BdrpU8.rst b/Misc/NEWS.d/next/Library/2026-04-27-20-15-54.gh-issue-149083.BdrpU8.rst
new file mode 100644
index 00000000000000..7ad81616802116
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2026-04-27-20-15-54.gh-issue-149083.BdrpU8.rst
@@ -0,0 +1,2 @@
+:data:`dataclasses.MISSING` and :data:`dataclasses.KW_ONLY` are now
+instances of :class:`sentinel`.