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

Doc/library/pathlib.rst

@@ -564,10 +564,10 @@ Pure paths provide the following methods and properties:
       True
 
 
-.. method:: PurePath.relative_to(*other)
+.. method:: PurePath.relative_to(*other, walk_up=False)
 
    Compute a version of this path relative to the path represented by
-   *other*.  If it's impossible, ValueError is raised::
+   *other*.  If it's impossible, :exc:`ValueError` is raised::
 
       >>> p = PurePosixPath('/etc/passwd')
       >>> p.relative_to('/')
@@ -577,11 +577,33 @@ Pure paths provide the following methods and properties:
       >>> p.relative_to('/usr')
       Traceback (most recent call last):
         File "<stdin>", line 1, in <module>
-        File "pathlib.py", line 694, in relative_to
+        File "pathlib.py", line 941, in relative_to
-          .format(str(self), str(formatted)))
+          raise ValueError(error_message.format(str(self), str(formatted)))
-      ValueError: '/etc/passwd' is not in the subpath of '/usr' OR one path is relative and the other absolute.
+      ValueError: '/etc/passwd' is not in the subpath of '/usr' OR one path is relative and the other is absolute.
 
-   NOTE: This function is part of :class:`PurePath` and works with strings. It does not check or access the underlying file structure.
+When *walk_up* is False (the default), the path must start with *other*.
+   When the argument is True, ``..`` entries may be added to form the
+   relative path. In all other cases, such as the paths referencing
+   different drives, :exc:`ValueError` is raised.::
+
+      >>> p.relative_to('/usr', walk_up=True)
+      PurePosixPath('../etc/passwd')
+      >>> p.relative_to('foo', walk_up=True)
+      Traceback (most recent call last):
+        File "<stdin>", line 1, in <module>
+        File "pathlib.py", line 941, in relative_to
+          raise ValueError(error_message.format(str(self), str(formatted)))
+      ValueError: '/etc/passwd' is not on the same drive as 'foo' OR one path is relative and the other is absolute.
+
+   .. warning::
+      This function is part of :class:`PurePath` and works with strings.
+      It does not check or access the underlying file structure.
+      This can impact the *walk_up* option as it assumes that no symlinks
+      are present in the path; call :meth:`~Path.resolve` first if
+      necessary to resolve symlinks.
+
+   .. versionadded:: 3.12
+      The *walk_up* argument (old behavior is the same as ``walk_up=False``).
 
 
 .. method:: PurePath.with_name(name)

Doc/whatsnew/3.12.rst

@@ -149,6 +149,11 @@ pathlib
   all file or directory names within them, similar to :func:`os.walk`.
   (Contributed by Stanislav Zmiev in :gh:`90385`.)
 
+* Add *walk_up* optional parameter to :meth:`pathlib.PurePath.relative_to`
+  to allow the insertion of ``..`` entries in the result; this behavior is
+  more consistent with :func:`os.path.relpath`.
+  (Contributed by Domenico Ragusa in :issue:`40358`.)
+
 dis
 ---
 

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)
+        other_drv, other_root, other_parts = self._parse_args(other)
-        if to_root:
+        if other_root:
-            to_abs_parts = [to_drv, to_root] + to_parts[1:]
+            other_abs_parts = [other_drv, other_root] + other_parts[1:]
         else:
-            to_abs_parts = to_parts
+            other_abs_parts = other_parts
-        n = len(to_abs_parts)
+        num_parts = len(other_abs_parts)
-        cf = self._flavour.casefold_parts
+        casefold = self._flavour.casefold_parts
-        if (root or drv) if n == 0 else cf(abs_parts[:n]) != cf(to_abs_parts):
+        num_common_parts = 0
-            formatted = self._format_parsed_parts(to_drv, to_root, to_parts)
+        for part, other_part in zip(casefold(abs_parts), casefold(other_abs_parts)):
-            raise ValueError("{!r} is not in the subpath of {!r}"
+            if part != other_part:
-                    " OR one path is relative and the other is absolute."
+                break
-                             .format(str(self), str(formatted)))
+            num_common_parts += 1
-        return self._from_parsed_parts('', root if n == 1 else '',
+        if walk_up:
-                                       abs_parts[n:])
+            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:
+            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.

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

Misc/ACKS

@@ -1440,6 +1440,7 @@ Pierre Quentel
 Brian Quinlan
 Anders Qvist
 Thomas Rachel
+Domenico Ragusa
 Ram Rachum
 Jeffrey Rackauckas
 Jérôme Radix

Misc/NEWS.d/next/Library/2020-04-30-02-15-08.bpo-40358.A4ygqe.rst

@@ -0,0 +1 @@
+Add walk_up argument in :meth:`pathlib.PurePath.relative_to`.