pathlib.PurePath.relative_to(walk_up=True) mishandles '..' components

[barneygale]

pathlib.PurePath.relative_to(other, walk_up=True) doesn't handle '..' segments in its other argument correctly:

>>> from pathlib import PurePath, Path
>>> Path.cwd()
PosixPath('/home/barney/projects/cpython')
>>> PurePath('a/b').relative_to('a/..', walk_up=True)
PurePosixPath('../b')
# expected: ValueError (ideal world: 'a/b')
>>> PurePath('a/b').relative_to('a/../..', walk_up=True)
PurePosixPath('../../b')
# expected: ValueError (ideal world: 'cpython/a/b')

PurePath objects do not know the current working directory, nor can they safely eliminate .. segments without resolving symlinks, so I think raising ValueError is the only reasonable thing to do.

Linked PRs

• gh-105002: [pathlib] Fix relative_to with walk_up=True using ".." #107014
• [3.12] gh-105002: [pathlib] Fix relative_to with walk_up=True using ".." (GH-107014) #107315

[barneygale]

@domragusa FYI! A patch like this should do the trick:

diff --git a/Lib/pathlib.py b/Lib/pathlib.py
index 8cb5279d73..e6bf388f1f 100644
--- a/Lib/pathlib.py
+++ b/Lib/pathlib.py
@@ -602,6 +602,8 @@ 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 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")