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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"`enum.Flag.__contains__` changed behavior since python 3.12","articleBody":"# Bug report\n\n### Bug description:\n\nI noticed some strange behavior with `enum.Flag` an the `__contains__` method in Python 3.12/3.13, as shown in the following examples.\n\n**Problem 1: Behavior changes at runtime**\nIn the following code snippet the first print statement returns `False`, which is expected, since `3` is not a member of `Weekday`.  However, the second print statement returns `True`, which is unexpected, since `3` is still not a member of `Weekday`.\n\n```python\nimport enum\n\nclass Weekday(enum.Flag):\n    MONDAY = 1\n    TUESDAY = 2\n    WEDNESDAY = 4\n    THURSDAY = 8\n    FRIDAY = 16\n    SATURDAY = 32\n    SUNDAY = 64\n\nprint(f\"{3 in Weekday}\") # =\u003e False (expected)\n_ = Weekday.MONDAY | Weekday.TUESDAY\nprint(f\"{3 in Weekday}\") # =\u003e True (not expected)\n```\n\n\n\n**Problem 2: Behavior is not comparable to Python 3.11**\nSince Python 3.12 the behavior of `Enum.__contains__` has changed, so that it is possible to compare not only with an enum-member, but also with non-enum-members (see [here](https://github.com/python/cpython/pull/93298) or [here](https://docs.python.org/3/library/enum.html#enum.EnumType.__contains__)). So with Python 3.11 the code above will raise an `TypeError: unsupported operand type(s) for 'in': 'int' and 'EnumType'`. There you have to change the code to the following. But this in turn always produces unexpected behavior in Python 3.12/3.13.\n\n```python\nimport enum\n\nclass Weekday(enum.Flag):\n    MONDAY = 1\n    TUESDAY = 2\n    WEDNESDAY = 4\n    THURSDAY = 8\n    FRIDAY = 16\n    SATURDAY = 32\n    SUNDAY = 64\n\nprint(f\"{Weekday(3) in Weekday}\") # =\u003e Python 3.11: False (expected)    Python 3.12/3.13: True (not expected)\n_ = Weekday.MONDAY | Weekday.TUESDAY\nprint(f\"{Weekday(3) in Weekday}\") # =\u003e Python 3.11: False (expected)    Python 3.12/3.13: True (not expected)\n```\n\n**Conclusion**\nI would have expected that in all cases the result is `False`, but since Python 3.12 it gets very strange...\n\n### CPython versions tested on:\n\n3.12, 3.13\n\n### Operating systems tested on:\n\nWindows\n\n\u003c!-- gh-linked-prs --\u003e\n### Linked PRs\n* gh-131053\n* gh-131167\n* gh-131232\n\u003c!-- /gh-linked-prs --\u003e\n","author":{"url":"https://github.com/timrid","@type":"Person","name":"timrid"},"datePublished":"2025-03-10T14:29:22.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":6},"url":"https://github.com/131045/cpython/issues/131045"}