add compatibility shim for enum34 backport

[ethanfurman]

BPO	26981
Nosy	@warsaw, @ericvsmith, @ethanfurman
Superseder	bpo-26988: Add AutoNumberedEnum to stdlib

^{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 2016-08-20.14:21:28.451>
created_at = <Date 2016-05-08.23:45:21.159>
labels = ['type-bug']
title = 'add compatibility shim for enum34 backport'
updated_at = <Date 2016-08-20.15:57:12.847>
user = 'https://github.com/ethanfurman'

bugs.python.org fields:

activity = <Date 2016-08-20.15:57:12.847>
actor = 'ethan.furman'
assignee = 'ethan.furman'
closed = True
closed_date = <Date 2016-08-20.14:21:28.451>
closer = 'ethan.furman'
components = []
creation = <Date 2016-05-08.23:45:21.159>
creator = 'ethan.furman'
dependencies = []
files = []
hgrepos = []
issue_num = 26981
keywords = []
message_count = 9.0
messages = ['265174', '265175', '265176', '272072', '273191', '273214', '273216', '273220', '273221']
nosy_count = 6.0
nosy_names = ['barry', 'eric.smith', 'eli.bendersky', 'SilentGhost', 'ethan.furman', 'python-dev']
pr_nums = []
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = '26988'
type = 'behavior'
url = 'https://bugs.python.org/issue26981'
versions = ['Python 3.6']

[ethanfurman]

_sunder_ methods are reserved for internal Enum use, and if the user tries to use any an exception is raised.

Consequently, the enum34 backport uses __order__ instead of _order_ to specify a definition order for its members.

I would like to do two things:

1. add _order_ to the stdlib version

2. ensure the actual definition order is the same as what is specified
   in _order_

[ericvsmith]

Doesn't namedtuple use _fields (as opposed to _fields_) for a similar purpose? Would Enum using _order be more consistent with that?

[ethanfurman]

It would, but that's not what we did.

Currently we have _name_ and _value_; the backport uses __order__ because I didn't want the stdlib version having a hairball when someone tried to use a 2.7 Enum in 3.4 and it didn't occur to me at the time to just add and ignore _order_.

[ethanfurman]

Done in bpo-26988.

[ethanfurman]

bpo-26988 reverted. Need to add shim here.

[python-dev]

New changeset 10830e1b2966 by Ethan Furman in branch 'default':
bpo-26981: add _order_ compatibility shim to enum.Enum
https://hg.python.org/cpython/rev/10830e1b2966

[SilentGhost]

Ethan, in Misc/NEWS it says "compatibility ship"

[python-dev]

New changeset bdfd4c8384de by Ethan Furman in branch 'default':
bpo-26981: fix typo
https://hg.python.org/cpython/rev/bdfd4c8384de

[ethanfurman]

Thanks!