https://github.com/python/cpython/commit/16952218d0535904236e8a39851133688c9ce1f0
commit: 16952218d0535904236e8a39851133688c9ce1f0
branch: main
author: Jelle Zijlstra <[email protected]>
committer: JelleZijlstra <[email protected]>
date: 2026-04-29T19:39:29-07:00
summary:
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.
files:
A Misc/NEWS.d/next/Library/2026-04-27-20-15-54.gh-issue-149083.BdrpU8.rst
M Lib/dataclasses.py
M Lib/test/test_dataclasses/__init__.py
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`.
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