Bug report

In pathlib prior to Python 3.12, passing a PureWindowsPath to a PurePosixPath resulted in the Windows separator (\) being converted to the POSIX separator (/). However, in the current main branch the backslashes are preserved in the PurePosixPath object.

Here is an example which illustrates this:

Python 3.12.0a7 (tags/v3.12.0a7:b861ba4, Apr  6 2023, 16:09:18) [Clang 10.0.0 ] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import pathlib
>>> print(pathlib.PurePosixPath(pathlib.PureWindowsPath(r"a\b\c")))
a\b\c
>>> print(pathlib.PurePosixPath(pathlib.PureWindowsPath(r"a\b\c")).as_posix())
a\b\c 

Python 3.11.2 (tags/v3.11.2:878ead1, Mar  9 2023, 16:26:59) [Clang 10.0.0 ] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import pathlib
>>> print(pathlib.PurePosixPath(pathlib.PureWindowsPath(r"a\b\c")))
a/b/c
>>> print(pathlib.PurePosixPath(pathlib.PureWindowsPath(r"a\b\c")).as_posix())
a/b/c 

(The behaviour is the same if using PosixPath or WindowsPath on the relevant platform; it's not specific to the "pure" variants.)

Before the recent refactoring of the module, passing one Path or PurePath object to another resulted in the _parts attribute being inspected. This was a list of the individual path elements (e.g. ['a', 'b', 'c'] for the path a\b\c). The _parts attribute was removed in GH-102476 and replaced with _tail, but with slightly different semantics.

The current code replaces any os.altsep in the path with os.sep, which for WindowsPath replaces / with \ but for PosixPath does nothing as there is no alternative separator. However, the following will produce a correct result:

Python 3.12.0a7+ (heads/main:bd2ed06, Apr 19 2023, 15:27:47) [GCC 9.4.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import pathlib
>>> pathlib.PurePosixPath._flavour.altsep = "\\"
>>> pathlib.PurePosixPath(pathlib.PureWindowsPath(r"a\b\c")).as_posix()
'a/b/c'

Thus I think the problem can be isolated to these lines here:

Your environment

• CPython versions tested on: 3.8, 3.11 and 3.12 (alpha 7 and HEAD)
• Operating system and architecture: Ubuntu 20.04 and Windows 10

Linked PRs

• GH-103631: Fix PurePosixPath(PureWindowsPath(...)) separator handling #104949
• [3.12] GH-103631: Fix PurePosixPath(PureWindowsPath(...)) separator handling (GH-104949) #104991