Dict order is now guaranteed, so add tests and doc for it

[rhettinger]

BPO	32337
Nosy	@rhettinger, @ned-deily, @stevendaprano, @methane, @zhangyangyu, @shangdahao, @miss-islington
PRs	bpo-32337: Update tutorial and documentation related with dict order. #4973 [3.7] bpo-32337: Update documentats about dict order (GH-4973) #6368

^{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 = None
closed_at = <Date 2018-04-04.17:36:05.905>
created_at = <Date 2017-12-15.18:14:10.450>
labels = ['3.8', '3.7', 'tests']
title = 'Dict order is now guaranteed, so add tests and doc for it'
updated_at = <Date 2018-04-04.17:36:05.903>
user = 'https://github.com/rhettinger'

bugs.python.org fields:

activity = <Date 2018-04-04.17:36:05.903>
actor = 'ned.deily'
assignee = 'none'
closed = True
closed_date = <Date 2018-04-04.17:36:05.905>
closer = 'ned.deily'
components = ['Tests']
creation = <Date 2017-12-15.18:14:10.450>
creator = 'rhettinger'
dependencies = []
files = []
hgrepos = []
issue_num = 32337
keywords = ['patch']
message_count = 11.0
messages = ['309060', '309114', '311490', '311585', '313624', '314916', '314917', '314918', '314920', '314930', '314943']
nosy_count = 7.0
nosy_names = ['rhettinger', 'ned.deily', 'steven.daprano', 'methane', 'xiang.zhang', 'shangdahao', 'miss-islington']
pr_nums = ['4973', '6368']
priority = None
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = None
url = 'https://bugs.python.org/issue32337'
versions = ['Python 3.7', 'Python 3.8']

[methane]

FYI, Builtin dict is tested for ordering already:

cpython/Lib/test/test_ordered_dict.py

Lines 646 to 662 in 13a6c09

	class CPythonBuiltinDictTests(unittest.TestCase):
	"""Builtin dict preserves insertion order.
	Reuse some of tests in OrderedDict selectively.
	"""
	module = builtins
	OrderedDict = dict
	for method in (
	"test_init test_update test_abc test_clear test_delitem " +
	"test_setitem test_detect_deletion_during_iteration " +
	"test_popitem test_reinsert test_override_update " +
	"test_highly_nested test_highly_nested_subclass " +
	"test_delitem_hash_collision ").split():
	setattr(CPythonBuiltinDictTests, method, getattr(OrderedDictTests, method))
	del method

[shangdahao]

Thanks inada, I removed the tests from the PR.

[zhangyangyu]

This part needs editing too, the text and example. https://docs.python.org/3/library/stdtypes.html#dictionary-view-objects

[shangdahao]

Thanks zhang, I have updated it in the PR.

[ned-deily]

Raymond, do you want to review the current state of the PR before 3.7.0b4? Otherwise, I'll probably ask that it be merged as is.

[stevendaprano]

I started to add this comment to bpo-33218 but Raymond closed that ticket while I was editing it.

In 3.6, dicts preserving insertion order remains an implementation detail that cannot be relied on. It only becomes a guarantee in 3.7.

Even in 3.7, I think that it remains best to think of dicts as fundamentally unordered. Dicts might preserve insertion order, but there's no interface for extracting the n-th item, or re-ordering the keys into some alternative order, and for the purposes of equality comparisons, the order is ignored.

I propose changing the description from "It is best to think of a dictionary as an unordered set of key: value pairs" to something like:

Dicts are an otherwise unordered set of key:value pairs
that remember the order keys were inserted.

and references to "arbitrary order" should be replaced by "insertion order". But by no means should we suggest that dicts are fundamentally orderable (sortable).

[methane]

I started to add this comment to bpo-33218 but Raymond closed that ticket while I was editing it.

Sorry, I closed it, not Raymond.