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Title: gh-121291: Respect mixin bitwise-operator overrides on Flag subclasses by SomSamantray · Pull Request #155862 · python/cpython · GitHub

Open Graph Title: gh-121291: Respect mixin bitwise-operator overrides on Flag subclasses by SomSamantray · Pull Request #155862 · python/cpython

X Title: gh-121291: Respect mixin bitwise-operator overrides on Flag subclasses by SomSamantray · Pull Request #155862 · python/cpython

Description: EnumType.__new__ (Lib/enum.py) installs Flag's __or__/__and__/__xor__/__ror__/__rand__/__rxor__/__invert__ onto every Flag subclass whenever the name isn't in the subclass's own class body — but that check never looked at what a mixin base actually resolved to, so class MyFlag(Mixin, Flag) silently lost the mixin's own operator overrides to Flag's defaults. The __repr__/__str__/__format__/__reduce_ex__ block a few lines above already handles this correctly (checking whether the class's real MRO-resolved method is still the type default before overwriting); this gives the Flag-operator loop the same MRO-aware check. The check also has to distinguish a mixin's own override from a mixed-in data type's raw operator (e.g. int.__or__ for IntFlag) — an earlier version of this fix compared only against Flag's own method and broke every IntFlag combination (Color.RED | Color.BLUE silently returned a plain int instead of a boxed IntFlag), caught by running the full test_enum suite including the Doc/howto/enum.rst doctests. Lib/test/test_enum.py gets a regression test in both OldTestFlag and OldTestIntFlag covering: a mixin's override winning over Flag's default (forward and reflected forms), dunders the mixin didn't touch still getting Flag's own, and the IntFlag/int regression case. Full suite: 1091 tests pass, 0 failures. One known scope boundary, called out during review: when the overriding mixin is itself the class selected as member_type (e.g. a custom int subclass that also overrides __or__), the identity check still can't tell that apart from the raw data-type operator — the same limitation the pre-existing __repr__/__str__ block already has. Fixing that would need a heavier MRO/__dict__-walk redesign touching both blocks; out of scope here. Fixes #121291. Issue: gh-121291

Open Graph Description: EnumType.__new__ (Lib/enum.py) installs Flag's __or__/__and__/__xor__/__ror__/__rand__/__rxor__/__invert__ onto every Flag subclass whenever the name isn't in the subclass's own class b...

X Description: EnumType.__new__ (Lib/enum.py) installs Flag's __or__/__and__/__xor__/__ror__/__rand__/__rxor__/__invert__ onto every Flag subclass whenever the name isn't in the subclass's...

Opengraph URL: https://github.com/python/cpython/pull/155862

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b3fbdb6 gh-121291: respect mixin-defined bitwise operators on Flag subclasses SomSamantray Aug 15, 2026 https://github.com/python/cpython/pull/155862/commits/b3fbdb6206442f3505c63c0695ff9d135051564e
eea42a8 gh-121291: add regression tests for Flag/IntFlag mixin operators SomSamantray Aug 15, 2026 https://github.com/python/cpython/pull/155862/commits/eea42a81b159fab2412eb0bef41f45b064d5fa55
e51888d gh-121291: add NEWS entry for Flag mixin operator fix SomSamantray Aug 15, 2026 https://github.com/python/cpython/pull/155862/commits/e51888d4278dc87f01599b07951b0fad3d44a05a
c8cbf38 test: reuse existing self.Color fixture in IntFlag mixin test SomSamantray Aug 15, 2026 https://github.com/python/cpython/pull/155862/commits/c8cbf38c9ea03e7c528ae8417171f3bc145024a7
a908741 test: cover __ror__/__rand__/__rxor__ in Flag mixin operator test SomSamantray Aug 15, 2026 https://github.com/python/cpython/pull/155862/commits/a9087414e6f5187fe1406f1ad88563b41d455b77
070ec61 Merge branch 'main' into gh-121291-enum-flag-mixin-operators SomSamantray Aug 29, 2026 https://github.com/python/cpython/pull/155862/commits/070ec61c706a30ce60fffa3fe751db82680229e8
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