pathlib ABCs: restore relative_to() and is_relative_to()

Lib/pathlib/__init__.py

@@ -490,16 +490,19 @@ def relative_to(self, other, *, walk_up=False):
         """
         if not hasattr(other, 'with_segments'):
             other = self.with_segments(other)
-        for step, path in enumerate(chain([other], other.parents)):
+        parts = []
+        for path in chain([other], other.parents):
             if path == self or path in self.parents:
                 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:
+                parts.append('..')
         else:
             raise ValueError(f"{str(self)!r} and {str(other)!r} have different anchors")
-        parts = ['..'] * step + self._tail[len(path._tail):]
+        parts.extend(self._tail[len(path._tail):])
         return self._from_parsed_parts('', '', parts)
 
     def is_relative_to(self, other):

Lib/pathlib/types.py

@@ -234,6 +234,33 @@ def parents(self):
             parent = split(path)[0]
         return tuple(parents)
 
+    def relative_to(self, other, *, walk_up=False):
+        """Return the relative path to another path identified by the passed
+        arguments.  If the operation is not possible (because this is not
+        related to the other path), raise ValueError.
+
+        The *walk_up* parameter controls whether `..` may be used to resolve
+        the path.
+        """
+        parts = []
+        for path in (other,) + other.parents:
+            if self.is_relative_to(path):
+                break
+            elif not walk_up:
+                raise ValueError(f"{self!r} is not in the subpath of {other!r}")
+            elif path.name == '..':
+                raise ValueError(f"'..' segment in {other!r} cannot be walked")
+            else:
+                parts.append('..')
+        else:
+            raise ValueError(f"{self!r} and {other!r} have different anchors")
+        return self.with_segments(*parts, *self.parts[len(path.parts):])
+
+    def is_relative_to(self, other):
+        """Return True if the path is relative to another path or False.
+        """
+        return other == self or other in self.parents
+
     def full_match(self, pattern):
         """
         Return True if this path matches the given glob-style pattern. The

Lib/test/test_pathlib/test_join.py

@@ -354,6 +354,61 @@ def test_with_suffix(self):
         self.assertRaises(ValueError, P('a/b').with_suffix, '.d/.')
         self.assertRaises(TypeError, P('a/b').with_suffix, None)
 
+    def test_relative_to(self):
+        P = self.cls
+        p = P('a/b')
+        self.assertEqual(p.relative_to(P('')), P('a', 'b'))
+        self.assertEqual(p.relative_to(P('a')), P('b'))
+        self.assertEqual(p.relative_to(P('a/b')), P(''))
+        self.assertEqual(p.relative_to(P(''), walk_up=True), P('a', 'b'))
+        self.assertEqual(p.relative_to(P('a'), walk_up=True), P('b'))
+        self.assertEqual(p.relative_to(P('a/b'), walk_up=True), P(''))
+        self.assertEqual(p.relative_to(P('a/c'), walk_up=True), P('..', 'b'))
+        self.assertEqual(p.relative_to(P('a/b/c'), walk_up=True), P('..'))
+        self.assertEqual(p.relative_to(P('c'), walk_up=True), P('..', 'a', 'b'))
+        self.assertRaises(ValueError, p.relative_to, P('c'))
+        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)
+        class Q(self.cls):
+            __eq__ = object.__eq__
+            __hash__ = object.__hash__
+        q = Q('a/b')
+        self.assertTrue(q.relative_to(q))
+        self.assertRaises(ValueError, q.relative_to, Q(''))
+        self.assertRaises(ValueError, q.relative_to, Q('a'))
+        self.assertRaises(ValueError, q.relative_to, Q('a'), walk_up=True)
+        self.assertRaises(ValueError, q.relative_to, Q('a/b'))
+        self.assertRaises(ValueError, q.relative_to, Q('c'))
+
+    def test_is_relative_to(self):
+        P = self.cls
+        p = P('a/b')
+        self.assertTrue(p.is_relative_to(P('')))
+        self.assertTrue(p.is_relative_to(P('a')))
+        self.assertTrue(p.is_relative_to(P('a/b')))
+        self.assertFalse(p.is_relative_to(P('c')))
+        self.assertFalse(p.is_relative_to(P('a/b/c')))
+        self.assertFalse(p.is_relative_to(P('a/c')))
+        self.assertFalse(p.is_relative_to(P('/a')))
+        class Q(self.cls):
+            __eq__ = object.__eq__
+            __hash__ = object.__hash__
+        q = Q('a/b')
+        self.assertTrue(q.is_relative_to(q))
+        self.assertFalse(q.is_relative_to(Q('')))
+        self.assertFalse(q.is_relative_to(Q('a')))
+        self.assertFalse(q.is_relative_to(Q('a/b')))
+        self.assertFalse(q.is_relative_to(Q('c')))
+
 
 class LexicalPathJoinTest(JoinTestBase, unittest.TestCase):
     cls = LexicalPath