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CC:@pablogsal

Thanks very much for taking a look at this

I think the exception message is wrong here:

>>>PurePath("a/b/c").relative_to("a/b/..",walk_up=False)Traceback (mostrecentcalllast):File"<stdin>",line1,in<module>File"/home/barney/projects/cpython/Lib/pathlib.py",line637,inrelative_toraiseValueError(f"'..' segment in{str(other)!r} cannot be walked")ValueError:'..'segmentin'a/b/..'cannotbewalked

When we setwalk_up=False (the default), we shouldn't get an error about the '..' segment not being walkable, because we haven't asked to walk it.

Something like this might work better:

diff --git a/Lib/pathlib.py b/Lib/pathlib.pyindex 8ff4d4ea19..c83cf3d2ef 100644--- a/Lib/pathlib.py+++ b/Lib/pathlib.py@@ -633,10 +633,12 @@ def relative_to(self, other, /, *_deprecated, walk_up=False):         for step, path in enumerate([other] + list(other.parents)):             if self.is_relative_to(path):                 break+            elif not walk_up:+                raise ValueError(f"{str(self)!r} is not in the subpath of {str(other)!r}")+            elif path.name == '..':+                raise ValueError(f"'..' segment in {str(other)!r} cannot be walked")         else:             raise ValueError(f"{str(self)!r} and {str(other)!r} have different anchors")-        if step and not walk_up:-            raise ValueError(f"{str(self)!r} is not in the subpath of {str(other)!r}")         parts = ['..'] * step + self._tail[len(path._tail):]         return self.with_segments(*parts)

Thank you@barneygale for the fast reponse! Absolutely makes sense, I have applied your requested changes.

Thanks for making that change!

The bug seems to be reproducible with absolute paths too:

>>>PurePath('/a/b').relative_to('/a/..',walk_up=True)PurePosixPath('../b')

So it would be good to add a test case for that, and adjust the news entry to remove the mention of relative paths.

Also, the bug only occurs when users setwalk_up=True. I think that's important to mention in the news entry.

Thank you, I appreciate these suggestions!

I have modified the news entry to better reflect the changes.

Also, for the testing: You were right, sadly I always usedother as relative path in the new test asserts. Now maybe a little bit overdone because for both relative and absolute test cases the I have done some asserts for theother to be relative, absolute, with and withoutwalk_up=True, but hopefully now all things are covered.

Thanks@kukovecz for the PR, and@barneygale for merging it 🌮🎉.. I'm working now to backport this PR to: 3.12.
🐍🍒⛏🤖

GH-107315 is a backport of this pull request to the3.12 branch.

@kukovecz thanks again for this. By the way, we always squash PRs on merge, so there's no need to force-push. It can make PRs a little harder to follow for reviewers.

Thanks@barneygale for the quick round of reviews and suggestions along the way.

I was not sure of your settings / policies about updating the PR so I was trying to keep the history acceptable, manually. Maybe I should have read the contributor guide more carefully. I will take extra care next time not make the review harder that it needs to be.

No worries, it didn't make this review any harder! Just a tip for any future PRs you might make :-)