Lazy creation of __dict__ in OrderedDict

[serhiy-storchaka]

BPO	25949
Nosy	@rhettinger, @jcea, @ericsnowcurrently, @serhiy-storchaka
Files	odict___dict__.patch

^{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/serhiy-storchaka'
closed_at = <Date 2016-02-08.14:40:06.098>
created_at = <Date 2015-12-25.12:48:08.359>
labels = ['extension-modules', 'type-feature']
title = 'Lazy creation of __dict__ in OrderedDict'
updated_at = <Date 2017-04-25.01:18:03.564>
user = 'https://github.com/serhiy-storchaka'

bugs.python.org fields:

activity = <Date 2017-04-25.01:18:03.564>
actor = 'jcea'
assignee = 'serhiy.storchaka'
closed = True
closed_date = <Date 2016-02-08.14:40:06.098>
closer = 'serhiy.storchaka'
components = ['Extension Modules']
creation = <Date 2015-12-25.12:48:08.359>
creator = 'serhiy.storchaka'
dependencies = []
files = ['41419']
hgrepos = []
issue_num = 25949
keywords = ['patch']
message_count = 4.0
messages = ['256988', '257044', '258593', '259849']
nosy_count = 6.0
nosy_names = ['rhettinger', 'jcea', 'python-dev', 'eric.snow', 'serhiy.storchaka', 'Winterflower']
pr_nums = []
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue25949'
versions = ['Python 3.6']

[serhiy-storchaka]

For now OrderedDict always creates an empty dict for __dict__.

>>> from collections import OrderedDict
>>> import gc
>>> gc.get_referents(OrderedDict())
[{}]
>>> class OD(OrderedDict): pass
... 
>>> gc.get_referents(OD())
[<class '__main__.OD'>, {}]

But dict subclasses (as well as most other classes) create an empty dict for __dict__ only if needed.

>>> class D(dict): pass
... 
>>> d = D()
>>> gc.get_referents(d)
[<class '__main__.D'>]
>>> d.__dict__
{}
>>> gc.get_referents(d)
[{}, <class '__main__.D'>]

This allows to save CPU time for dictionary creation and a memory (144 bytes on 32-bit, twice as much on 64-bit).

Proposed patch makes __dict__ in OrderedDict to be created only if needed.

[Winterflower]

Hi Serhiy,
I tried to see whether the patch's unit test in test_ordered_dict.py would fail when the changes to odictobject.c were not applied and it did not.
The code change to test_ordered_dict.py does not appear to test the fact that a dict is not automatically created when an ordered dict is instantiated (?).

[serhiy-storchaka]

The patch have no visible effect, except lesser memory consumption. The latter is hard to measure in tests. Additional tests just ensure that the patch doesn't break existing behavior.

[python-dev]

New changeset caab6b356a9e by Serhiy Storchaka in branch 'default':
Issue bpo-25949: __dict__ for an OrderedDict instance is now created only when
https://hg.python.org/cpython/rev/caab6b356a9e