the pathlib.Path.parents[-1] didn't receive the self._parts

[ramwin]

Bug report
the pathlib.Path.parents[-1] didn't receive the self._parts
example:

[#6#wangx@manjaro-5800:~] $ python
Python 3.10.4 (main, Mar 23 2022, 23:05:40) [GCC 11.2.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> from pathlib import Path
>>> Path("/home/wangx").parents[-1]
PosixPath('/')
>>> Path("/home/wangx").parents[-1] == Path("/")
False
>>> Path("/home/wangx").parents[1] == Path("/")
True
>>> Path("/home/wangx").parents[1]
PosixPath('/')
>>> 

source code in pathlib.py:

 422 class _PathParents(Sequence):                                                   
 423     """This object provides sequence-like access to the logical ancestors       
 424     of a path.  Don't try to construct it yourself."""                          
 425     __slots__ = ('_pathcls', '_drv', '_root', '_parts')                         
 426                                                                                 
 427     def __init__(self, path):                                                   
 428         # We don't store the instance to avoid reference cycles                 
 429         self._pathcls = type(path)                                              
 430         self._drv = path._drv                                                   
 431         self._root = path._root                                                 
 432         self._parts = path._parts                                               
 433                                                                                 
 434     def __len__(self):                                                          
 435         if self._drv or self._root:                                             
 436             return len(self._parts) - 1                                         
 437         else:                                                                   
 438             return len(self._parts)                                             
 439                                                                                 
 440     def __getitem__(self, idx):                                                 
 441         if isinstance(idx, slice):                                              
 442             return tuple(self[i] for i in range(*idx.indices(len(self))))       
 443                                                                                 
 444         if idx >= len(self) or idx < -len(self):                                
 445             raise IndexError(idx)                                               
 446         return self._pathcls._from_parsed_parts(self._drv, self._root,          
 447                                                 self._parts[:-idx - 1])

when the idx is -1, -idx - 1 = 0. so the self._parts[: -idx -1 ] is empty. hence then the parents[-1] != parents[2] because the _parts is different.

Your environment

• CPython versions tested on: 3.10.4 and 3.12.0a0(main branch)
• Operating system and architecture: Manjaro Linux 21.2.6 Linux manjaro-5800 5.4.195-1-MANJARO #1 SMP PREEMPT Wed May 18 09:23:31 UTC 2022 x86_64 GNU/Linux

[barneygale]

This looks like a bug in 79d2e62 - negative indices into parents don't work properly with absolute paths.

[akulakov]

Seems to be a duplicate of #89577 (which doesn't have a PR)

[Edit: after looking through the comments on that issue, @MojoVampire came up with the same fix.]

[brettcannon]

#93480 (review)
#93479 (review)

[AA-Turner]

#93273 has been merged and backported.

A