gh-84538: add strict argument to pathlib.PurePath.relative_to (GH-19813)

By default, :meth:`pathlib.PurePath.relative_to` doesn't deal with paths that are not a direct prefix of the other, raising an exception in that instance. This change adds a *walk_up* parameter that can be set to allow for using ``..`` to calculate the relative path.

example:
```
>>> p = PurePosixPath('/etc/passwd')
>>> p.relative_to('/etc')
PurePosixPath('passwd')
>>> p.relative_to('/usr')
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "pathlib.py", line 940, in relative_to
    raise ValueError(error_message.format(str(self), str(formatted)))
ValueError: '/etc/passwd' does not start with '/usr'
>>> p.relative_to('/usr', strict=False)
PurePosixPath('../etc/passwd')
```


https://bugs.python.org/issue40358

Automerge-Triggered-By: GH:brettcannon

2 files changed, 115 insertions, 15 deletions

diff --git a/Lib/pathlib.py b/Lib/pathlib.py
index 0ea621c..1498ce0 100644
--- a/Lib/pathlib.py
+++ b/Lib/pathlib.py
@@ -626,10 +626,13 @@ class PurePath(object):
         return self._from_parsed_parts(self._drv, self._root,
                                        self._parts[:-1] + [name])
 
-    def relative_to(self, *other):
+    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
-        a subpath of the other path), raise ValueError.
+        related to the other path), raise ValueError.
+
+        The *walk_up* parameter controls whether `..` may be used to resolve
+        the path.
         """
         # For the purpose of this method, drive and root are considered
         # separate parts, i.e.:
@@ -644,20 +647,35 @@ class PurePath(object):
             abs_parts = [drv, root] + parts[1:]
         else:
             abs_parts = parts
-        to_drv, to_root, to_parts = self._parse_args(other)
-        if to_root:
-            to_abs_parts = [to_drv, to_root] + to_parts[1:]
+        other_drv, other_root, other_parts = self._parse_args(other)
+        if other_root:
+            other_abs_parts = [other_drv, other_root] + other_parts[1:]
+        else:
+            other_abs_parts = other_parts
+        num_parts = len(other_abs_parts)
+        casefold = self._flavour.casefold_parts
+        num_common_parts = 0
+        for part, other_part in zip(casefold(abs_parts), casefold(other_abs_parts)):
+            if part != other_part:
+                break
+            num_common_parts += 1
+        if walk_up:
+            failure = root != other_root
+            if drv or other_drv:
+                failure = casefold([drv]) != casefold([other_drv]) or (failure and num_parts > 1)
+            error_message = "{!r} is not on the same drive as {!r}"
+            up_parts = (num_parts-num_common_parts)*['..']
         else:
-            to_abs_parts = to_parts
-        n = len(to_abs_parts)
-        cf = self._flavour.casefold_parts
-        if (root or drv) if n == 0 else cf(abs_parts[:n]) != cf(to_abs_parts):
-            formatted = self._format_parsed_parts(to_drv, to_root, to_parts)
-            raise ValueError("{!r} is not in the subpath of {!r}"
-                    " OR one path is relative and the other is absolute."
-                             .format(str(self), str(formatted)))
-        return self._from_parsed_parts('', root if n == 1 else '',
-                                       abs_parts[n:])
+            failure = (root or drv) if num_parts == 0 else num_common_parts != num_parts
+            error_message = "{!r} is not in the subpath of {!r}"
+            up_parts = []
+        error_message += " OR one path is relative and the other is absolute."
+        if failure:
+            formatted = self._format_parsed_parts(other_drv, other_root, other_parts)
+            raise ValueError(error_message.format(str(self), str(formatted)))
+        path_parts = up_parts + abs_parts[num_common_parts:]
+        new_root = root if num_common_parts == 1 else ''
+        return self._from_parsed_parts('', new_root, path_parts)
 
     def is_relative_to(self, *other):
         """Return True if the path is relative to another path or False.
diff --git a/Lib/test/test_pathlib.py b/Lib/test/test_pathlib.py
index f324177..3b1f302 100644
--- a/Lib/test/test_pathlib.py
+++ b/Lib/test/test_pathlib.py
@@ -640,13 +640,29 @@ class _BasePurePathTest(object):
         self.assertEqual(p.relative_to('a/'), P('b'))
         self.assertEqual(p.relative_to(P('a/b')), P())
         self.assertEqual(p.relative_to('a/b'), P())
+        self.assertEqual(p.relative_to(P(), walk_up=True), P('a/b'))
+        self.assertEqual(p.relative_to('', walk_up=True), P('a/b'))
+        self.assertEqual(p.relative_to(P('a'), walk_up=True), P('b'))
+        self.assertEqual(p.relative_to('a', walk_up=True), P('b'))
+        self.assertEqual(p.relative_to('a/', walk_up=True), P('b'))
+        self.assertEqual(p.relative_to(P('a/b'), walk_up=True), P())
+        self.assertEqual(p.relative_to('a/b', walk_up=True), P())
+        self.assertEqual(p.relative_to(P('a/c'), walk_up=True), P('../b'))
+        self.assertEqual(p.relative_to('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('a/b/c', walk_up=True), P('..'))
+        self.assertEqual(p.relative_to(P('c'), walk_up=True), P('../a/b'))
+        self.assertEqual(p.relative_to('c', walk_up=True), P('../a/b'))
         # With several args.
         self.assertEqual(p.relative_to('a', 'b'), P())
+        self.assertEqual(p.relative_to('a', 'b', walk_up=True), P())
         # Unrelated paths.
         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('/'), 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'))
@@ -655,6 +671,19 @@ class _BasePurePathTest(object):
         self.assertEqual(p.relative_to('/a/'), P('b'))
         self.assertEqual(p.relative_to(P('/a/b')), P())
         self.assertEqual(p.relative_to('/a/b'), P())
+        self.assertEqual(p.relative_to(P('/'), walk_up=True), P('a/b'))
+        self.assertEqual(p.relative_to('/', walk_up=True), P('a/b'))
+        self.assertEqual(p.relative_to(P('/a'), walk_up=True), P('b'))
+        self.assertEqual(p.relative_to('/a', walk_up=True), P('b'))
+        self.assertEqual(p.relative_to('/a/', walk_up=True), P('b'))
+        self.assertEqual(p.relative_to(P('/a/b'), walk_up=True), P())
+        self.assertEqual(p.relative_to('/a/b', walk_up=True), P())
+        self.assertEqual(p.relative_to(P('/a/c'), walk_up=True), P('../b'))
+        self.assertEqual(p.relative_to('/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('/a/b/c', walk_up=True), P('..'))
+        self.assertEqual(p.relative_to(P('/c'), walk_up=True), P('../a/b'))
+        self.assertEqual(p.relative_to('/c', walk_up=True), P('../a/b'))
         # Unrelated paths.
         self.assertRaises(ValueError, p.relative_to, P('/c'))
         self.assertRaises(ValueError, p.relative_to, P('/a/b/c'))
@@ -662,6 +691,8 @@ class _BasePurePathTest(object):
         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(''), walk_up=True)
+        self.assertRaises(ValueError, p.relative_to, P('a'), walk_up=True)
 
     def test_is_relative_to_common(self):
         P = self.cls
@@ -1124,6 +1155,16 @@ class PureWindowsPathTest(_BasePurePathTest, unittest.TestCase):
         self.assertEqual(p.relative_to('c:foO/'), P('Bar'))
         self.assertEqual(p.relative_to(P('c:foO/baR')), P())
         self.assertEqual(p.relative_to('c:foO/baR'), P())
+        self.assertEqual(p.relative_to(P('c:'), walk_up=True), P('Foo/Bar'))
+        self.assertEqual(p.relative_to('c:', walk_up=True), P('Foo/Bar'))
+        self.assertEqual(p.relative_to(P('c:foO'), walk_up=True), P('Bar'))
+        self.assertEqual(p.relative_to('c:foO', walk_up=True), P('Bar'))
+        self.assertEqual(p.relative_to('c:foO/', walk_up=True), P('Bar'))
+        self.assertEqual(p.relative_to(P('c:foO/baR'), walk_up=True), P())
+        self.assertEqual(p.relative_to('c:foO/baR', walk_up=True), P())
+        self.assertEqual(p.relative_to(P('C:Foo/Bar/Baz'), walk_up=True), P('..'))
+        self.assertEqual(p.relative_to(P('C:Foo/Baz'), walk_up=True), P('../Bar'))
+        self.assertEqual(p.relative_to(P('C:Baz/Bar'), walk_up=True), P('../../Foo/Bar'))
         # Unrelated paths.
         self.assertRaises(ValueError, p.relative_to, P())
         self.assertRaises(ValueError, p.relative_to, '')
@@ -1134,6 +1175,13 @@ class PureWindowsPathTest(_BasePurePathTest, unittest.TestCase):
         self.assertRaises(ValueError, p.relative_to, P('C:/Foo'))
         self.assertRaises(ValueError, p.relative_to, P('C:Foo/Bar/Baz'))
         self.assertRaises(ValueError, p.relative_to, P('C:Foo/Baz'))
+        self.assertRaises(ValueError, p.relative_to, P(), walk_up=True)
+        self.assertRaises(ValueError, p.relative_to, '', walk_up=True)
+        self.assertRaises(ValueError, p.relative_to, P('d:'), walk_up=True)
+        self.assertRaises(ValueError, p.relative_to, P('/'), walk_up=True)
+        self.assertRaises(ValueError, p.relative_to, P('Foo'), walk_up=True)
+        self.assertRaises(ValueError, p.relative_to, P('/Foo'), walk_up=True)
+        self.assertRaises(ValueError, p.relative_to, P('C:/Foo'), walk_up=True)
         p = P('C:/Foo/Bar')
         self.assertEqual(p.relative_to(P('c:')), P('/Foo/Bar'))
         self.assertEqual(p.relative_to('c:'), P('/Foo/Bar'))
@@ -1146,6 +1194,20 @@ class PureWindowsPathTest(_BasePurePathTest, unittest.TestCase):
         self.assertEqual(p.relative_to('c:/foO/'), P('Bar'))
         self.assertEqual(p.relative_to(P('c:/foO/baR')), P())
         self.assertEqual(p.relative_to('c:/foO/baR'), P())
+        self.assertEqual(p.relative_to(P('c:'), walk_up=True), P('/Foo/Bar'))
+        self.assertEqual(p.relative_to('c:', walk_up=True), P('/Foo/Bar'))
+        self.assertEqual(str(p.relative_to(P('c:'), walk_up=True)), '\\Foo\\Bar')
+        self.assertEqual(str(p.relative_to('c:', walk_up=True)), '\\Foo\\Bar')
+        self.assertEqual(p.relative_to(P('c:/'), walk_up=True), P('Foo/Bar'))
+        self.assertEqual(p.relative_to('c:/', walk_up=True), P('Foo/Bar'))
+        self.assertEqual(p.relative_to(P('c:/foO'), walk_up=True), P('Bar'))
+        self.assertEqual(p.relative_to('c:/foO', walk_up=True), P('Bar'))
+        self.assertEqual(p.relative_to('c:/foO/', walk_up=True), P('Bar'))
+        self.assertEqual(p.relative_to(P('c:/foO/baR'), walk_up=True), P())
+        self.assertEqual(p.relative_to('c:/foO/baR', walk_up=True), P())
+        self.assertEqual(p.relative_to('C:/Baz', walk_up=True), P('../Foo/Bar'))
+        self.assertEqual(p.relative_to('C:/Foo/Bar/Baz', walk_up=True), P('..'))
+        self.assertEqual(p.relative_to('C:/Foo/Baz', walk_up=True), P('../Bar'))
         # Unrelated paths.
         self.assertRaises(ValueError, p.relative_to, P('C:/Baz'))
         self.assertRaises(ValueError, p.relative_to, P('C:/Foo/Bar/Baz'))
@@ -1156,6 +1218,12 @@ class PureWindowsPathTest(_BasePurePathTest, unittest.TestCase):
         self.assertRaises(ValueError, p.relative_to, P('/'))
         self.assertRaises(ValueError, p.relative_to, P('/Foo'))
         self.assertRaises(ValueError, p.relative_to, P('//C/Foo'))
+        self.assertRaises(ValueError, p.relative_to, P('C:Foo'), walk_up=True)
+        self.assertRaises(ValueError, p.relative_to, P('d:'), walk_up=True)
+        self.assertRaises(ValueError, p.relative_to, P('d:/'), walk_up=True)
+        self.assertRaises(ValueError, p.relative_to, P('/'), walk_up=True)
+        self.assertRaises(ValueError, p.relative_to, P('/Foo'), walk_up=True)
+        self.assertRaises(ValueError, p.relative_to, P('//C/Foo'), walk_up=True)
         # UNC paths.
         p = P('//Server/Share/Foo/Bar')
         self.assertEqual(p.relative_to(P('//sErver/sHare')), P('Foo/Bar'))
@@ -1166,11 +1234,25 @@ class PureWindowsPathTest(_BasePurePathTest, unittest.TestCase):
         self.assertEqual(p.relative_to('//sErver/sHare/Foo/'), P('Bar'))
         self.assertEqual(p.relative_to(P('//sErver/sHare/Foo/Bar')), P())
         self.assertEqual(p.relative_to('//sErver/sHare/Foo/Bar'), P())
+        self.assertEqual(p.relative_to(P('//sErver/sHare'), walk_up=True), P('Foo/Bar'))
+        self.assertEqual(p.relative_to('//sErver/sHare', walk_up=True), P('Foo/Bar'))
+        self.assertEqual(p.relative_to('//sErver/sHare/', walk_up=True), P('Foo/Bar'))
+        self.assertEqual(p.relative_to(P('//sErver/sHare/Foo'), walk_up=True), P('Bar'))
+        self.assertEqual(p.relative_to('//sErver/sHare/Foo', walk_up=True), P('Bar'))
+        self.assertEqual(p.relative_to('//sErver/sHare/Foo/', walk_up=True), P('Bar'))
+        self.assertEqual(p.relative_to(P('//sErver/sHare/Foo/Bar'), walk_up=True), P())
+        self.assertEqual(p.relative_to('//sErver/sHare/Foo/Bar', walk_up=True), P())
+        self.assertEqual(p.relative_to(P('//sErver/sHare/bar'), walk_up=True), P('../Foo/Bar'))
+        self.assertEqual(p.relative_to('//sErver/sHare/bar', walk_up=True), P('../Foo/Bar'))
         # Unrelated paths.
         self.assertRaises(ValueError, p.relative_to, P('/Server/Share/Foo'))
         self.assertRaises(ValueError, p.relative_to, P('c:/Server/Share/Foo'))
         self.assertRaises(ValueError, p.relative_to, P('//z/Share/Foo'))
         self.assertRaises(ValueError, p.relative_to, P('//Server/z/Foo'))
+        self.assertRaises(ValueError, p.relative_to, P('/Server/Share/Foo'), walk_up=True)
+        self.assertRaises(ValueError, p.relative_to, P('c:/Server/Share/Foo'), walk_up=True)
+        self.assertRaises(ValueError, p.relative_to, P('//z/Share/Foo'), walk_up=True)
+        self.assertRaises(ValueError, p.relative_to, P('//Server/z/Foo'), walk_up=True)
 
     def test_is_relative_to(self):
         P = self.cls