[Enum] standardize format() behavior

[ethanfurman]

BPO	43945
Nosy	@warsaw, @ethanfurman
PRs	bpo-43945: [Enum] Deprecate non-standard mixin format() behavior #25649 bpo-43945: [Enum] reduce scope of new format() behavior #26752 [3.10] bpo-43945: [Enum] reduce scope of new format() behavior (GH-26752) #26791

^{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 2021-06-18.21:28:01.096>
created_at = <Date 2021-04-26.23:04:32.335>
labels = ['type-bug', '3.10', '3.11']
title = '[Enum] standardize format() behavior'
updated_at = <Date 2021-06-29.16:52:05.269>
user = 'https://github.com/ethanfurman'

bugs.python.org fields:

activity = <Date 2021-06-29.16:52:05.269>
actor = 'barry'
assignee = 'ethan.furman'
closed = True
closed_date = <Date 2021-06-18.21:28:01.096>
closer = 'ethan.furman'
components = []
creation = <Date 2021-04-26.23:04:32.335>
creator = 'ethan.furman'
dependencies = []
files = []
hgrepos = []
issue_num = 43945
keywords = ['patch']
message_count = 5.0
messages = ['391995', '392028', '395904', '396083', '396097']
nosy_count = 2.0
nosy_names = ['barry', 'ethan.furman']
pr_nums = ['25649', '26752', '26791']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue43945'
versions = ['Python 3.10', 'Python 3.11']

[ethanfurman]

Currently, an enum with a mixed-in data type, such as IntEnum, will use that data type's __format__ -- unless the user provides their own __str__, in which case the str() of the enum member will be used in the format() call.

This behavior will be deprecated in 3.10, and in 3.12 the default __format__ will use the default __str__, which is the standard behavior for Python objects

For those that were relying on, for example,

    class Color(IntEnum):
        RED = 1

f'{Color.RED}' # -> '2'

They will need to add ":d" to ensure the integer output:

f'{Color.RED:d}'

This change does work now.

[ethanfurman]

New changeset 5987b8c by Ethan Furman in branch 'master':
bpo-43945: [Enum] Deprecate non-standard mixin format() behavior (GH-25649)
5987b8c

[ethanfurman]

Thinking about this some more I am partially reversing course. The idea behind IntEnum and IntFlag is to be, as close as possible, drop-in replacements for existing integer-based constants. If the format() of those two change then they become much less attractive.

So it won't. Instead, any user-mixed enumerations will get the new behavior:

    class Grades(int, Enum):
        A = 5
        F = 0

will emit a DeprecationWarning now, and in 3.12 format(Grades.A) will product 'A' instead of '5'.

[ethanfurman]

New changeset f60b07a by Ethan Furman in branch 'main':
bpo-43945: [Enum] reduce scope of new format() behavior (GH-26752)
f60b07a

[ethanfurman]

New changeset 1b4addf by Ethan Furman in branch '3.10':
[3.10] bpo-43945: [Enum] reduce scope of new format() behavior (GH-26752)
1b4addf