[issue21041] pathlib.PurePath.parents rejects negative indexes

[Josh Rosenberg]

Josh Rosenberg  added the comment:

Negative indexing is broken for absolute paths, see #45414.

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[issue21041] pathlib.PurePath.parents rejects negative indexes

[Paul Ganssle]

Paul Ganssle  added the comment:


New changeset 79d2e62c008446fbbc6f264bb8a30e2d38b6ff58 by Yaroslav Pankovych in 
branch 'master':
Added support for negative indexes to PurePath.parents (GH-21799)
https://github.com/python/cpython/commit/79d2e62c008446fbbc6f264bb8a30e2d38b6ff58


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[issue21041] pathlib.PurePath.parents rejects negative indexes

[Yaroslav Pankovych]

Yaroslav Pankovych  added the comment:

Agree with that, it currently supports this behavior.

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[issue21041] pathlib.PurePath.parents rejects negative indexes

[Paul Ganssle]

Paul Ganssle  added the comment:

I think you may have confused my thoughts as to why this might be considered 
ambiguous with an actual suggestion for what the semantics should be.

I think that we should stick with `p.parents[x] == tuple(p.parents)[x]` for any 
valid value of `x`, which means that `p.parents[-1]` will always be `Path('.')` 
for any non-empty `p`.

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[issue21041] pathlib.PurePath.parents rejects negative indexes

[Yaroslav Pankovych]

Yaroslav Pankovych  added the comment:

And it looks like a special case, so "Special cases aren't special enough to 
break the rules."

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[issue21041] pathlib.PurePath.parents rejects negative indexes

[Yaroslav Pankovych]

Yaroslav Pankovych  added the comment:

That makes sense, but should we have this behaviour only for negative indices? 

We'll end up with something lie:

path.parents[len(path.parents) - 1] != path.parents[-1]

I think that is should be consistent regardless of negative/positive indices.

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[issue21041] pathlib.PurePath.parents rejects negative indexes

[Paul Ganssle]

Paul Ganssle  added the comment:

I am not seeing any compelling reasons to avoid supporting negative indexes 
*or* slices here.

If I had to guess about the confusing semantics of negative indices, I would 
guess it's the fact that the index in the -1 position for a non-empty Path will 
always be `Path('.')`. Since that's not terribly useful, it might be reasonable 
to have negative indices start counting at `len(p)-2`.

That said, I don't think this is a big deal, and I think we have more 
speculation on why this was avoided in the first place than we have actual 
objections to changing it, so I vote for changing it.

I think our best option is to say that the semantics of indexing `.parents` 
should be the same as indexing the result of casting it to a tuple, so this 
should be true:

p = Path(x)
assert p.parents[y] == tuple(p.parents)[y]

For all values of `x` and `y`.

I've gone ahead and changed the version support matrix to 3.10 only, since I 
think that this was a deliberate choice and we should be considering this an 
enhancement rather than a bugfix. That said, I'll admit that it's on the 
borderline — the semantics of sequences are unambiguous (see, which says that 
sequences support both slices and negative indices: 
https://docs.python.org/3/library/stdtypes.html#typesseq ), and PEP 428 
explicitly says that .parents returns a "an immutable sequence of the path's 
logical ancestors": 
https://www.python.org/dev/peps/pep-0428/#sequence-like-access . So if someone 
is motivated to try and make the case that this is a bugfix that could be 
backported to earlier supported versions, I won't stand in their way.

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versions:  -Python 3.5, Python 3.6, Python 3.7, Python 3.8, Python 3.9

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[issue21041] pathlib.PurePath.parents rejects negative indexes

[Yaroslav Pankovych]

Yaroslav Pankovych  added the comment:

Any thoughts about that folks? It's a pretty old bug, let's decide smth for it.

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[issue21041] pathlib.PurePath.parents rejects negative indexes

[Yaroslav Pankovych]

Yaroslav Pankovych  added the comment:

Here's possible fix: https://github.com/python/cpython/pull/21799

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[issue21041] pathlib.PurePath.parents rejects negative indexes

[Yaroslav Pankovych]

Yaroslav Pankovych  added the comment:

That's kinda weird for python. I mean, in regular list/etc if I need the last 
element, I'd normally do list[-1], but here to get the last parent, I need to 
actually know how many parents do I have.

So now, I can do something like this:

>>> parents_count = len(path.parents) - 1
>>> path.parents[parents_count]
PosixPath('.')

--
message_count: 16.0 -> 17.0
nosy: +ypank
nosy_count: 10.0 -> 11.0
pull_requests: +20938
stage:  -> patch review
versions: +Python 3.10, Python 3.5, Python 3.6, Python 3.7, Python 3.9
pull_request: https://github.com/python/cpython/pull/21799

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[issue21041] pathlib.PurePath.parents rejects negative indexes

[Serhiy Storchaka]

Serhiy Storchaka  added the comment:

Maxwell, in your case a more correct and obvious way is

Path('/tmp') in filepath.parents

although it may be not very efficient. Using the is_relative_to() method may be 
more efficient and obvious.

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[issue21041] pathlib.PurePath.parents rejects negative indexes

[Maxwell Ballenger]

Maxwell Ballenger  added the comment:

Use case: I want to see if a Path is a descendent of /tmp.

if filepath.parents[-2] == Path('tmp'):

turns into

if filepath.parents[len(filepath.parents)-2] == Path('tmp'):

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[issue21041] pathlib.PurePath.parents rejects negative indexes

[Julin]

Julin  added the comment:

Can't this be implemented? This is something that a user would expect. 
Intuitive. And it looks as if it is an easy change to make that doesn't disturb 
anything else.

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[issue21041] pathlib.PurePath.parents rejects negative indexes

[victorg]

victorg  added the comment:

Allow negative indexes that could be usefull. 

Example: to compare 2 or more Path, if they come from the same top directory

from pathlib import Path
a = Path("/a/testpy/cpython/config.log")
b = Path("/b/testpy/cpython/config.log")
c = Path("/a/otherfolder/text.txt")
print(f"a.parents[-2] == b.parents[-2] -> {a.parents[-2] == b.parents[-2]}") # 
False
print(f"a.parents[-2] == c.parents[-2] -> {a.parents[-2] == c.parents[-2]}") # 
True 
# index = -2 because -1 is "/"

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Added file: https://bugs.python.org/file48949/allowNegativeIndexParents.patch

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[issue21041] pathlib.PurePath.parents rejects negative indexes

[Joshua Cannon]

Joshua Cannon  added the comment:

> it may explain why negative indices or slices were initially not implemented: 
> It already looks like the result of a slice.

Sure the values of the sequence could be thought of as being increasingly 
smaller slices of some other sequence, however I don't think it changes the 
fact that "parents" is a sequence, and sequences have well-defined semantics 
for negative indices and slices. Semantics which people expect, and have to 
find smelly workarounds for.

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[issue21041] pathlib.PurePath.parents rejects negative indexes

[Julien Palard]

Julien Palard  added the comment:

I checked conversation in #7951, tells about an ambiguity because it could be 
an index from a sequence or a key for a dict, like {-1: "foo"}.

Here there is no such confusion.

Confusion *may* arrise from the fact that it's not composed of parts, but more 
like it's already sliced, I mean it does NOT look like:

   ['/', 'home', 'mdk', 'clones', 'python']

It's more like:

   ['/home/mdk/clones/python', '/home/mdk/clones', '/home/mdk', '/home', '/']


In fact I'd say it behave more like a function call than a sequence access, I 
read:

   pathlib.Path.cwd().parents[1]

a bit like:

   pathlib.Path.cwd().parents(go_down=1)

It may explain why negative indices or slices were initially not implemented: 
It already looks like the result of a slice.

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[issue21041] pathlib.PurePath.parents rejects negative indexes

[Joshua Cannon]

Joshua Cannon  added the comment:

I created issue35498 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.

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[issue21041] pathlib.PurePath.parents rejects negative indexes

[Mark Lawrence]

Change by Mark Lawrence :


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[issue21041] pathlib.PurePath.parents rejects negative indexes

[Serhiy Storchaka]

Change by Serhiy Storchaka :


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type: behavior -> enhancement
versions: +Python 3.8 -Python 3.4, Python 3.5

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[issue21041] pathlib.PurePath.parents rejects negative indexes

[Akira Li]

Akira Li added the comment:

 #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]?

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[issue21041] pathlib.PurePath.parents rejects negative indexes

[Barry A. Warsaw]

Changes by Barry A. Warsaw ba...@python.org:


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[issue21041] pathlib.PurePath.parents rejects negative indexes

[Barry A. Warsaw]

Barry A. Warsaw added the comment:

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]

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[issue21041] pathlib.PurePath.parents rejects negative indexes

[akira]

akira added the comment:

 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.

[1]: 
https://docs.python.org/3/library/stdtypes.html#sequence-types-list-tuple-range

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[issue21041] pathlib.PurePath.parents rejects negative indexes

[Mark Lawrence]

Mark Lawrence added the comment:

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

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[issue21041] pathlib.PurePath.parents rejects negative indexes

[akira]

akira added the comment:

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?

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[issue21041] pathlib.PurePath.parents rejects negative indexes

[akira]

Changes by akira 4kir4...@gmail.com:


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type:  - behavior

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[issue21041] pathlib.PurePath.parents rejects negative indexes

[R. David Murray]

Changes by R. David Murray rdmur...@bitdance.com:


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[issue21041] pathlib.PurePath.parents rejects negative indexes

[R. David Murray]

R. David Murray added the comment:

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

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[issue21041] pathlib.PurePath.parents rejects negative indexes

[Antoine Pitrou]

Antoine Pitrou added the comment:

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.

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[issue21041] pathlib.PurePath.parents rejects negative indexes

[R. David Murray]

R. David Murray added the comment:

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.

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[issue21041] pathlib.PurePath.parents rejects negative indexes

[akira]

New submission from akira:

`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.

[1]: http://docs.python.org/3/library/pathlib#pathlib.PurePath.parents
[2]: 
http://docs.python.org/3/library/stdtypes.html#sequence-types-list-tuple-range

--
components: Library (Lib)
files: pathlib-parents-allow-negative-index.patch
keywords: patch
messages: 214642
nosy: akira
priority: normal
severity: normal
status: open
title: pathlib.PurePath.parents rejects negative indexes
versions: Python 3.4, Python 3.5
Added file: 
http://bugs.python.org/file34595/pathlib-parents-allow-negative-index.patch

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