need StrEnum in enum.py

[ethanfurman]

BPO	41816
Nosy	@warsaw, @terryjreedy, @ethanfurman, @serhiy-storchaka
PRs	bpo-41816: add StrEnum #22337 bpo-41816: StrEnum.__str__ is str.__str__ #22362

^{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 2020-09-22.20:01:30.869>
created_at = <Date 2020-09-19.19:20:58.192>
labels = ['type-feature', 'library', '3.10']
title = 'need StrEnum in enum.py'
updated_at = <Date 2020-09-22.20:01:30.868>
user = 'https://github.com/ethanfurman'

bugs.python.org fields:

activity = <Date 2020-09-22.20:01:30.868>
actor = 'ethan.furman'
assignee = 'ethan.furman'
closed = True
closed_date = <Date 2020-09-22.20:01:30.869>
closer = 'ethan.furman'
components = ['Library (Lib)']
creation = <Date 2020-09-19.19:20:58.192>
creator = 'ethan.furman'
dependencies = []
files = []
hgrepos = []
issue_num = 41816
keywords = ['patch']
message_count = 5.0
messages = ['377186', '377294', '377321', '377347', '377348']
nosy_count = 5.0
nosy_names = ['barry', 'terry.reedy', 'eli.bendersky', 'ethan.furman', 'serhiy.storchaka']
pr_nums = ['22337', '22362']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue41816'
versions = ['Python 3.10']

[ethanfurman]

Due to the nature of str, if an Enum tries to mixin the str type, all of it's members will be strings -- even if they didn't start out that way:

  class MyStrEnum(str, Enum):
     tuple = 'oops',
     okay = 'correct'

  >>> list(MyStrEnum)
  [<MyStrEnum.tuple: "('oops',)">, MyStrEnum.okay: 'correct'>]

The StrEnum class will have a check to ensure that each value was already a string, or can be converted to a string via

  str(bytes, encoding, errors)

[ethanfurman]

New changeset 0063ff4 by Ethan Furman in branch 'master':
bpo-41816: add StrEnum (GH-22337)
0063ff4

[ethanfurman]

From Serhiy Storchaka:
---------------------
The only exception is StrEnum -- overriding __str__ of str
subclass may be not safe. Some code will call str() implicitly, other
will read the string content of the object directly, and they will be
different.