Make class definition namespace ordered by default

[ericsnowcurrently]

BPO	24254
Nosy	@jcea, @ncoghlan, @vstinner, @ned-deily, @ethanfurman, @ericsnowcurrently, @berkerpeksag
Dependencies	bpo-16991: Add OrderedDict written in C
Files	odict-class-definition-namespace.diff just-default-to-odict.diff deforder.diff deforder-with-tp-slot.diff

^{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/ericsnowcurrently'
closed_at = <Date 2016-12-07.00:56:52.742>
created_at = <Date 2015-05-21.05:34:48.307>
labels = ['interpreter-core', 'deferred-blocker', 'type-bug']
title = 'Make class definition namespace ordered by default'
updated_at = <Date 2017-05-17.18:21:22.630>
user = 'https://github.com/ericsnowcurrently'

bugs.python.org fields:

activity = <Date 2017-05-17.18:21:22.630>
actor = 'jcea'
assignee = 'eric.snow'
closed = True
closed_date = <Date 2016-12-07.00:56:52.742>
closer = 'ncoghlan'
components = ['Interpreter Core']
creation = <Date 2015-05-21.05:34:48.307>
creator = 'eric.snow'
dependencies = ['16991']
files = ['39446', '42834', '43169', '43739']
hgrepos = []
issue_num = 24254
keywords = ['patch']
message_count = 35.0
messages = ['243729', '243767', '243902', '244055', '244056', '244060', '244061', '244075', '244077', '265434', '266957', '266981', '267183', '267319', '269846', '270431', '270517', '273881', '273887', '273898', '274441', '275188', '275337', '275347', '275580', '275585', '275605', '275606', '275622', '275624', '275656', '275660', '275662', '282574', '282583']
nosy_count = 9.0
nosy_names = ['jcea', 'ncoghlan', 'vstinner', 'ned.deily', 'ethan.furman', 'fijall', 'python-dev', 'eric.snow', 'berker.peksag']
pr_nums = []
priority = 'deferred blocker'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue24254'
versions = ['Python 3.6']

[ericsnowcurrently]

Here's a work-in-progress patch that uses OrderedDict as the default class definition namespace (instead of dict). While the actual class namespace is a dict, the definition order is preserved on a new attribute on class objects: __definition_order__. This is useful for cases such as class decorators that would make use of the definition order to post-process the class. This is something that Guido okay'ed a while back.

It may be a stretch to get this into 3.5, but the final change should be pretty small. It also depends on the C implementation of OrderedDict (see issue bpo-16991) which has not landed and may not make it in time.

Note that the patch is still a bit buggy, partly due to bugs in the C OrderedDict patch. Also, the attached patch is based off the C OrderedDict patch, so I expect reitveld won't be able to handle it. The code on which the patch is based may be found in the "class-namespace" branch of https://pypi.python.org/pypi/cyordereddict.

[ericsnowcurrently]

Here's a patch that drops adding definition_order. You can get the same effect by adding __definition_order__ = list(locals()) at the bottom of your class definition. The benefit of having __definition_order__ is that the information is automatically available for later use rather than forcing the class author to add the above line in order to benefit from external introspection/manipulation. The downside is that every syntax-defined class will have the extra attribute with the list of definition order, which might be an unnecessary waste, even if not a huge one.

Ultimately, I'd prefer to keep __definition_order__.

[ncoghlan]

types.prepare_class() also needs to be updated to use OrderedDict() by default.

[ericsnowcurrently]

Per discussion on python-dev, I'm tabling the __definition_order__ part to 3.6. I'll open a thread on python-ideas on it later and open a new issue here if I get a positive response.

So this issue is just about making OrderedDict the default namespace type for class definitions.

[vstinner]

It may be a stretch to get this into 3.5, but the final change should be pretty small.

Changing the default type of class dictionaries is a huge change. IMO it should be deferred to Python 3.6.

[ericsnowcurrently]

This is not about changing the default type for class dictionaries. It is only for changing the default type used during class definition. Essentially, we are just changing the type of what is returned from type.__prepare__. cls.dict will remain a dict.

[ncoghlan]

This is *not* about changing the default type of class dictionaries (which
I agree would be far too large a change to make without a PEP), it's only
about changing the ephemeral evaluation namespace used to execute the class
body.

[ericsnowcurrently]

Thanks for pointing out types.prepare_class. I've updated it.

[ericsnowcurrently]

I've moved this to 3.6. Small as the patch might be, there just isn't enough urgency to warrant making use of an exception to get it into 3.5. If __definition_order__ were still on the table then I'd probably still push for 3.5. :)

[ericsnowcurrently]

Here's a refresh of the patch that only sets the default definition namespace (and does not introduce __definition_order__).