Pretty printing sorting for set and frozenset instances

[danilobellini]

BPO	27495
Nosy	@freddrake, @rhettinger, @stevendaprano, @serhiy-storchaka, @danilobellini, @bbonenfant
PRs	gh-71682: Make pprint.pformat always order sets #22977
Files	pprint_small_set_sorted.patch: Sorts small set/frozenset on pprint and adds test_sorted_set_frozenset saferepr_collections.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 = None
created_at = <Date 2016-07-12.11:30:29.623>
labels = ['type-bug', '3.8', '3.9', '3.10', '3.7', 'library']
title = 'Pretty printing sorting for set and frozenset instances'
updated_at = <Date 2020-10-25.20:05:08.751>
user = 'https://github.com/danilobellini'

bugs.python.org fields:

activity = <Date 2020-10-25.20:05:08.751>
actor = 'bbonenfant'
assignee = 'serhiy.storchaka'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2016-07-12.11:30:29.623>
creator = 'danilo.bellini'
dependencies = []
files = ['43698', '43864']
hgrepos = []
issue_num = 27495
keywords = ['patch']
message_count = 8.0
messages = ['270237', '270246', '270325', '270326', '270332', '271155', '279412', '369795']
nosy_count = 7.0
nosy_names = ['fdrake', 'rhettinger', 'steven.daprano', 'SilentGhost', 'serhiy.storchaka', 'danilo.bellini', 'bbonenfant']
pr_nums = ['22977']
priority = 'normal'
resolution = None
stage = 'patch review'
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue27495'
versions = ['Python 3.7', 'Python 3.8', 'Python 3.9', 'Python 3.10']

[danilobellini]

The pprint pretty printer in Python 3 sorts sets/frozensets only if their length don't fit in one single line/row for the given width, else it was just leaving repr(my_set_instance) alone, like:

>>> import string, pprint
>>> pprint.pprint(set(string.digits))
{'7', '5', '2', '4', '1', '9', '6', '3', '0', '8'}

That order is quite random in Python 3.2+. But on Python 2.6 and 2.7, the result is shown as:
set(['0', '1', '2', '3', '4', '5', '6', '7', '8', '9'])

So for using pprint in doctests (or anything alike) with sets/frozensets, the pretty printer isn't as useful in Python 3 than it is in Python 2. The pprint tests for non-nested set/frozenset were only using some small ranges for testing. I've written a patch to solve that.

[freddrake]

+1

It could reasonably be argued that not sorting is a bug for already-released 3.x versions.

[rhettinger]

+1 for treating this as a bug fix.

[serhiy-storchaka]

Since pprint() now supports all standard collections, I think it is worth to support them in saferepr() too. I have written an implementation and am finishing writing tests.

[danilobellini]

Wouldn't a fix for all standard collections be a fix for Python 3.5+, therefore another issue? http://bugs.python.org/issue23870

This issue is about sets/frozensets Python 3.2+, and I'm pretty sure it's backwards compatible, as I don't think any code running on Python 3.2.6 would depend on pprint randomness (how could?). Also, a multiline pprint would sort (tested with Python 3.2.6):

>>> pprint.pprint(set(string.digits), width=7)
{'0',
 '1',
 '2',
 '3',
 '4',
 '5',
 '6',
 '7',
 '8',
 '9'}

I see no reason to see a fix to this inconsistent behavior (sorting on multiline, not sorting on single line) as an enhancement just for a new Python 3.6 version. Besides being backwards compatible, the test_pprint was really verifying the order on set(range(n)) for small n, something that is already sorted by set.__repr__ but appears in test_pprint, which make me think it was intended as a pretty printer test, not as a set.__repr__ test.

[serhiy-storchaka]

Following patch adds support of all standard collections in pprint.saferepr().

[stevendaprano]

There seems to be consensus that this should be treated as a bug fix, not a new feature. Could this still make it into 3.6 even though it missed the first beta?