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gh-149083: Convert dataclasses.MISSING and dataclasses.KW_ONLY to sentinels #149086

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Merged
JelleZijlstra merged 1 commit into from
Apr 30, 2026
+7 −10
Merged

gh-149083: Convert dataclasses.MISSING and dataclasses.KW_ONLY to sentinels #149086

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13 changes: 4 additions & 9 deletions Lib/dataclasses.py
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Original file line number Diff line number Diff line change
Expand Up @@ -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.
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2 changes: 1 addition & 1 deletion Lib/test/test_dataclasses/__init__.py
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Expand Up @@ -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')
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@ericvsmith ericvsmith Apr 30, 2026

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I have no idea why the arguments were in the opposite order before, but changing them is a definite improvement!

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@ericvsmith ericvsmith Apr 30, 2026

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And yes, I realize that wasn't really the point of this change.


def test_dont_include_other_annotations(self):
@dataclass
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2 changes: 2 additions & 0 deletions Misc/NEWS.d/next/Library/2026-04-27-20-15-54.gh-issue-149083.BdrpU8.rst
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@@ -0,0 +1,2 @@
:data:`dataclasses.MISSING` and :data:`dataclasses.KW_ONLY` are now
instances of :class:`sentinel`.
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