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

It makes sense to raise an Error because ".." can not
be resolved and the current working directory is unknown.

2 files changed, 16 insertions, 2 deletions

diff --git a/Lib/pathlib.py b/Lib/pathlib.py
index 8ff4d4e..c83cf3d 100644
--- a/Lib/pathlib.py
+++ b/Lib/pathlib.py
@@ -633,10 +633,12 @@ class PurePath:
         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)
 
diff --git a/Lib/test/test_pathlib.py b/Lib/test/test_pathlib.py
index 78948e3..5789a93 100644
--- a/Lib/test/test_pathlib.py
+++ b/Lib/test/test_pathlib.py
@@ -693,8 +693,14 @@ class PurePathTest(unittest.TestCase):
         self.assertRaises(ValueError, p.relative_to, P('a/b/c'))
         self.assertRaises(ValueError, p.relative_to, P('a/c'))
         self.assertRaises(ValueError, p.relative_to, P('/a'))
+        self.assertRaises(ValueError, p.relative_to, P("../a"))
+        self.assertRaises(ValueError, p.relative_to, P("a/.."))
+        self.assertRaises(ValueError, p.relative_to, P("/a/.."))
         self.assertRaises(ValueError, p.relative_to, P('/'), walk_up=True)
         self.assertRaises(ValueError, p.relative_to, P('/a'), walk_up=True)
+        self.assertRaises(ValueError, p.relative_to, P("../a"), walk_up=True)
+        self.assertRaises(ValueError, p.relative_to, P("a/.."), walk_up=True)
+        self.assertRaises(ValueError, p.relative_to, P("/a/.."), walk_up=True)
         p = P('/a/b')
         self.assertEqual(p.relative_to(P('/')), P('a/b'))
         self.assertEqual(p.relative_to('/'), P('a/b'))
@@ -723,8 +729,14 @@ class PurePathTest(unittest.TestCase):
         self.assertRaises(ValueError, p.relative_to, P())
         self.assertRaises(ValueError, p.relative_to, '')
         self.assertRaises(ValueError, p.relative_to, P('a'))
+        self.assertRaises(ValueError, p.relative_to, P("../a"))
+        self.assertRaises(ValueError, p.relative_to, P("a/.."))
+        self.assertRaises(ValueError, p.relative_to, P("/a/.."))
         self.assertRaises(ValueError, p.relative_to, P(''), walk_up=True)
         self.assertRaises(ValueError, p.relative_to, P('a'), walk_up=True)
+        self.assertRaises(ValueError, p.relative_to, P("../a"), walk_up=True)
+        self.assertRaises(ValueError, p.relative_to, P("a/.."), walk_up=True)
+        self.assertRaises(ValueError, p.relative_to, P("/a/.."), walk_up=True)
 
     def test_is_relative_to_common(self):
         P = self.cls