IntEnum __format__ behavior can't be overridden through __str__

[thatneat]

BPO	37479
Nosy	@ericvsmith, @ethanfurman, @thatneat
PRs	bpo-37479: on Enum subclasses with mixins, __format__ uses overridden __str__ #14545 [3.8] bpo-37479: Enum - use correct __format__ (GH-14545) #22227 [3.7] bpo-37479: Enum - user correct __format__ (GH-14545) #22228

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = 'https://github.com/ethanfurman'
closed_at = <Date 2020-09-13.20:51:07.553>
created_at = <Date 2019-07-01.23:24:11.546>
labels = ['3.8', 'type-bug', 'library', '3.9']
title = "IntEnum __format__ behavior can't be overridden through __str__"
updated_at = <Date 2020-09-13.20:51:07.551>
user = 'https://github.com/thatneat'

bugs.python.org fields:

activity = <Date 2020-09-13.20:51:07.551>
actor = 'ethan.furman'
assignee = 'ethan.furman'
closed = True
closed_date = <Date 2020-09-13.20:51:07.553>
closer = 'ethan.furman'
components = ['Library (Lib)']
creation = <Date 2019-07-01.23:24:11.546>
creator = 'jason.curtis'
dependencies = []
files = []
hgrepos = []
issue_num = 37479
keywords = ['patch']
message_count = 7.0
messages = ['347089', '347090', '347094', '347096', '347291', '376848', '376851']
nosy_count = 3.0
nosy_names = ['eric.smith', 'ethan.furman', 'jason.curtis']
pr_nums = ['14545', '22227', '22228']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue37479'
versions = ['Python 3.8', 'Python 3.9']

[thatneat]

Combining int and Enum, as with enum.IntEnum results in a class where __str__ cannot be effectively overridden. For example:

from enum import IntEnum

class myIntEnum(IntEnum):
    x = 1
    
    def __str__(self):
        return 'aaaaAAAa'
    
f'{myIntEnum.x}, {str(myIntEnum.x)}'

Expected output:
'aaaaAAAa, aaaaAAAa'

Actual output:
'1, aaaaAAAa'

Overriding __str__ in this way works as expected if the inherited classes are int or Enum individually. However, it does not work when inheriting (int, Enum) or when inheriting (intEnum).

Presumably this is a side effect of Enum's mixin behavior documented at https://docs.python.org/3/library/enum.html#others and it is possibly related to https://bugs.python.org/issue18264 .

[thatneat]

I mistyped - __str__ can be overridden effectively, but __format__ has to be overridden instead or overridden also to get the desired f-string behavior.

[thatneat]

related cPython code:

cpython/Lib/enum.py

Line 626 in 19a1e1e

if self._member_type_ is object:

If we're in a mixed-in class but the class has overridden __str__(), we should probably use str(self) instead of self._value_.

[ericvsmith]

Note that this isn't really related to f-strings, except that they use the __format__ protocol, as does str.__format__.

>>> format(myIntEnum.x)
'1'

[ethanfurman]

New changeset 2f19e82 by Ethan Furman (thatneat) in branch 'master':
bpo-37479: on Enum subclasses with mixins, __format__ uses overridden __str__ (GH-14545)
2f19e82

[ethanfurman]

New changeset 38c8d39 by Ethan Furman in branch '3.8':
[3.8] bpo-37479: Enum - use correct __format__ (GH-14545)
38c8d39

[ethanfurman]

Thank you, Jason!