enum.CONFORM behavior breaks backwards compatibility

[benburrill]

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

[sunmy2019]

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

[sunmy2019]

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

[benburrill]

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

[ethanfurman]

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