pathlib.PurePath.parents rejects negative indexes

[4kir4]

BPO	21041
Nosy	@warsaw, @pitrou, @bitdancer, @4kir4, @serhiy-storchaka, @MojoVampire, @JulienPalard, @pganssle, @thejcannon, @ju-sh, @VictorGob, @maxballenger, @ypankovych
PRs	bpo-21041: Add negative indexing to pathlib path parents #21799
Files	pathlib-parents-allow-negative-index.patch: the fix and tests allowNegativeIndexParents.patch: the fix w/o tests

^{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 2020-11-23.20:06:38.142>
created_at = <Date 2014-03-23.22:16:51.197>
labels = ['type-feature', 'library', '3.10']
title = 'pathlib.PurePath.parents rejects negative indexes'
updated_at = <Date 2021-10-08.18:12:25.910>
user = 'https://github.com/4kir4'

bugs.python.org fields:

activity = <Date 2021-10-08.18:12:25.910>
actor = 'josh.r'
assignee = 'none'
closed = True
closed_date = <Date 2020-11-23.20:06:38.142>
closer = 'p-ganssle'
components = ['Library (Lib)']
creation = <Date 2014-03-23.22:16:51.197>
creator = 'akira'
dependencies = []
files = ['34595', '48949']
hgrepos = []
issue_num = 21041
keywords = ['patch']
message_count = 26.0
messages = ['214642', '214709', '214716', '214717', '215746', '223048', '223054', '223059', '225503', '332008', '352281', '352322', '363337', '363743', '373269', '373281', '375099', '375100', '375788', '381452', '381563', '381654', '381670', '381678', '381694', '403489']
nosy_count = 13.0
nosy_names = ['barry', 'pitrou', 'r.david.murray', 'akira', 'serhiy.storchaka', 'josh.r', 'mdk', 'p-ganssle', 'thejcannon', 'ju-sh', 'victorg', 'maxballenger', 'ypank']
pr_nums = ['21799']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue21041'
versions = ['Python 3.10']

[4kir4]

pathlib.PurePath.parents is a sequence 1 but it rejects negative indexes:

  >>> from pathlib import PurePath
  >>> PurePath('a/b/c').parents[-2]
  Traceback (most recent call last):
  ...
  IndexError: -2

Sequences in Python interpret negative indexes as len(seq) + i 2

I've included the patch that fixes the issue and adds corresponding tests. No documentation changes are needed.

[bitdancer]

I think this is a doc bug. That object shouldn't be called a sequence, since it isn't one.

[pitrou]

Well, it is a sequence, it's just that it doesn't respect the convention about negative indices :-)

As to why they are disallowed, I don't remember exactly (!) but I think it's because the exact semantics would be confusing otherwise.

[bitdancer]

Which is exactly what I mean by saying it is not a sequence. It is 'sequence-like'. Kind of like email Messages are dict-like: they share many methods and behaviors, but the exact behaviors and semantics are different.

[4kir4]

From https://docs.python.org/3/glossary.html#term-sequence

An iterable which supports efficient element access using integer indices via the __getitem__() special method and defines a __len__() method that returns the length of the sequence.

.parents *is* a sequence. And it *is* confusing that it doesn't accept negative indexes -- that is how I've encountered the bug.

Antoine, could you elaborate on what are the negative consequences of negative indexes to justify breaking the expectations?

[warsaw]

Aren't negative indexes well defined in Python? E.g.

>>> p = Path('/tmp/tmp123/foo/bar/baz.xz')
>>> p.parents[len(p.parents)-2]
PosixPath('/tmp')

p.parents[-2] should == p.parents[len(p.parents)-2]

[4kir4]

Aren't negative indexes well defined in Python?

yes. I've provided the link to Python docs 1 in msg214642 that
explicitly defines the behavior:

If i or j is negative, the index is relative to the end of the string:
len(s) + i or len(s) + j is substituted. But note that -0 is still 0.

[BreamoreBoy]

bpo-7951 has an interesting debate on negative indexes that is possibly applicable here.

[4kir4]

bpo-7951 has an interesting debate on negative indexes that is possibly applicable here.

Mark could you point to a message that explains why p.parents[-2] is worse
than p.parents[len(p.parents)-2]?

[thejcannon]

I created bpo-35498 about .parents rejecting slices as well. (It was pointed out this discussion would probably decide that issue's fate)
I think that .parents looking like a duck, but not quacking like one isn't very pythonic.

Besides, the fact that p.parents[len(p.parents)-2] is allowed but p.parents[-2] is not just seems like extra steps. There's also list(p.parents)[-2], which is still not ideal. In either case, I'd imagine authors to put a comment like "PathLib .parents doesn't support negative indexes", which goes to show clients are expecting negative indices to work.

I see that this issue is several years old. I'm happy to shepherd it if it needs further contributions.