Description

benburrill

opened on Apr 8, 2023

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Encountered yet another breaking change for enums. There are two problems here:

• CONFORM doesn't "conform" to members that are partially unsupported
• CONFORM does not match the previous behavior for Flag. It used to be more similar to STRICT, but STRICT also doesn't allow partially unsupported members, while Flag in 3.10 did.

Example to reproduce:

import enum
class SkipFlag(enum.Flag):
    A = 1
    B = 2
    C = 4 | B

print(SkipFlag.C in (SkipFlag.A|SkipFlag.C))

class SkipIntFlag(enum.IntFlag):
    A = 1
    B = 2
    C = 4 | B

print(SkipIntFlag.C in (SkipIntFlag.A|SkipIntFlag.C))

print(SkipIntFlag(42).value)
print(SkipFlag(42).value)

In Python 3.10.6, this code outputs:

True
True
42
Traceback (most recent call last):
...
ValueError: 42 is not a valid SkipFlag

In Python 3.11.2:

False
True
42
2

Having such members as C is useful to create a flags that are distinct from B but always implies B.

In a previous comment bug report it was stated that the previous behavior of Flag was CONFORM:

Actually, won't need a deprecation period, since the proper default for Flag is CONFORM as that matches previous behavior.

Originally posted by @ethanfurman in #96865 (comment)

Clearly this is incorrect. The previous behavior was more similar to STRICT, but there appears to be no precise equivalent in Python 3.11 to the old behavior. Using STRICT in Python 3.11, SkipFlag.A | SkipFlag.C would fail to be created completely.

Linked PRs

• gh-103365: [Enum] STRICT boundary corrections #103494
• [3.11] gh-103365: [Enum] STRICT boundary corrections (GH-103494) #103513

Activity

1. benburrill

   added

   type-bugAn unexpected behavior, bug, or errorAn unexpected behavior, bug, or error

   on Apr 8, 2023
2. sobolevn

   assigned

   ethanfurman

   on Apr 8, 2023
3. sobolevn

   added

   stdlibStandard Library Python modules in the Lib/ directoryStandard Library Python modules in the Lib/ directory

   on Apr 8, 2023
4. sunmy2019 commented on Apr 8, 2023

   

   sunmy2019

   on Apr 8, 2023

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   With #24215, only single-bit values will be considered in Flag when using bitwise ops like &, |, ^, ~. Multi-bit values are only considered an alias.

   Thus SkipIntFlag.A | SkipIntFlag.C is actually SkipIntFlag.A | SkipIntFlag.B, since the 0b100 bit is discarded. But __contains__ will only consider its underlying values.

   You will need an element with value 4 inside of SkipFlag.

   This answers "why it behaves like this". Related Issues: #82431
5. sunmy2019 commented on Apr 8, 2023

   

   sunmy2019

   on Apr 8, 2023

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   Only single-bit values will be considered in Flag

   This is documented with an inconspicuous line.

   https://docs.python.org/3.10/library/enum.html#combining-members-of-flag
   https://docs.python.org/3.11/howto/enum.html#flag-and-intflag-minutia

   I think this should be added to the API reference here:
   https://docs.python.org/3/library/enum.html#enum.Flag
6. benburrill commented on Apr 8, 2023

   

   benburrill

   on Apr 8, 2023

   Author

   More actions

   https://docs.python.org/3.11/howto/enum.html#flag-and-intflag-minutia

   It's also documented there that the default for Flag in Python 3.11 is STRICT, but it's really CONFORM
7. ethanfurman

   added this to enum issueson Apr 12, 2023
8. ethanfurman

   moved this to In Progress in enum issueson Apr 12, 2023
9. ethanfurman

   added a commit that references this issue on Apr 13, 2023

   gh-103365: [Enum] STRICT boundary corrections (GH-103494)

   2194071
10. miss-islington

    added 2 commits that reference this issue on Apr 13, 2023

    pythongh-103365: [Enum] STRICT boundary corrections (pythonGH-103494)

    4598cf6

    gh-103365: [Enum] STRICT boundary corrections (GH-103494)

    804a973
11. carljm

    added a commit that references this issue on Apr 13, 2023

    Merge branch 'main' into superopt

    9384106
12. ethanfurman commented on Apr 13, 2023

    

    ethanfurman

    on Apr 13, 2023

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    @benburrill Thank you for the bug report. STRICT has been restored as the default for Flag, and the underlying issues fixed.

    Reacted by sunmy2019
13. ethanfurman

    closed this as completedon Apr 13, 2023
14. github-project-automation

    moved this from In Progress to Done in enum issueson Apr 13, 2023

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