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

1 files changed, 82 insertions, 0 deletions

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