pythonGH-113528: Deoptimise pathlib._abc.PurePathBase.relative_to()

Replace use of `_from_parsed_parts()` with `with_segments()` in
`PurePathBase.relative_to()`, and move the assignment of `_drv`, `_root`,
`_tail_cached` and `_str` slots into `PurePath.relative_to()`.

Also shuffle a deprecation warning into `PurePath.relative_to()` to
ensure it's raised at the right stack level, and do the same in
`is_relative_to()` for code consistency.

Lib/pathlib/__init__.py

@@ -9,6 +9,7 @@
 import ntpath
 import os
 import posixpath
+import warnings
 
 try:
     import pwd
@@ -164,6 +165,38 @@ def __ge__(self, other):
             return NotImplemented
         return self._parts_normcase >= other._parts_normcase
 
+    def relative_to(self, other, /, *_deprecated, 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.
+        """
+        if _deprecated:
+            msg = ("support for supplying more than one positional argument "
+                   "to pathlib.PurePath.relative_to() is deprecated and "
+                   "scheduled for removal in Python 3.14")
+            warnings.warn(msg, DeprecationWarning, stacklevel=2)
+            other = self.with_segments(other, *_deprecated)
+        path = _abc.PurePathBase.relative_to(self, other, walk_up=walk_up)
+        tail = path._raw_paths.copy()
+        path._str = self._format_parsed_parts('', '', tail) or '.'
+        path._drv = path._root = ''
+        path._tail_cached = tail
+        return path
+
+    def is_relative_to(self, other, /, *_deprecated):
+        """Return True if the path is relative to another path or False.
+        """
+        if _deprecated:
+            msg = ("support for supplying more than one argument to "
+                   "pathlib.PurePath.is_relative_to() is deprecated and "
+                   "scheduled for removal in Python 3.14")
+            warnings.warn(msg, DeprecationWarning, stacklevel=2)
+            other = self.with_segments(other, *_deprecated)
+        return _abc.PurePathBase.is_relative_to(self, other)
+
     def as_uri(self):
         """Return the path as a URI."""
         if not self.is_absolute():
@@ -230,7 +263,6 @@ def _unsupported(cls, method_name):
 
     def __init__(self, *args, **kwargs):
         if kwargs:
-            import warnings
             msg = ("support for supplying keyword arguments to pathlib.PurePath "
                    "is deprecated and scheduled for removal in Python {remove}")
             warnings._deprecated("pathlib.PurePath(**kwargs)", msg, remove=(3, 14))

Lib/pathlib/_abc.py

@@ -2,7 +2,6 @@
 import ntpath
 import posixpath
 import sys
-import warnings
 from _collections_abc import Sequence
 from errno import ENOENT, ENOTDIR, EBADF, ELOOP, EINVAL
 from itertools import chain
@@ -384,21 +383,15 @@ def with_suffix(self, suffix):
         else:
             raise ValueError(f"Invalid suffix {suffix!r}")
 
-    def relative_to(self, other, /, *_deprecated, walk_up=False):
+    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.
         """
-        if _deprecated:
-            msg = ("support for supplying more than one positional argument "
-                   "to pathlib.PurePath.relative_to() is deprecated and "
-                   "scheduled for removal in Python 3.14")
-            warnings.warn(msg, DeprecationWarning, stacklevel=2)
-            other = self.with_segments(other, *_deprecated)
-        elif not isinstance(other, PurePathBase):
+        if not isinstance(other, PurePathBase):
             other = self.with_segments(other)
         for step, path in enumerate(chain([other], other.parents)):
             if path == self or path in self.parents:
@@ -410,18 +403,12 @@ def relative_to(self, other, /, *_deprecated, walk_up=False):
         else:
             raise ValueError(f"{str(self)!r} and {str(other)!r} have different anchors")
         parts = ['..'] * step + self._tail[len(path._tail):]
-        return self._from_parsed_parts('', '', parts)
+        return self.with_segments(*parts)
 
-    def is_relative_to(self, other, /, *_deprecated):
+    def is_relative_to(self, other):
         """Return True if the path is relative to another path or False.
         """
-        if _deprecated:
-            msg = ("support for supplying more than one argument to "
-                   "pathlib.PurePath.is_relative_to() is deprecated and "
-                   "scheduled for removal in Python 3.14")
-            warnings.warn(msg, DeprecationWarning, stacklevel=2)
-            other = self.with_segments(other, *_deprecated)
-        elif not isinstance(other, PurePathBase):
+        if not isinstance(other, PurePathBase):
             other = self.with_segments(other)
         return other == self or other in self.parents
 
@@ -832,6 +819,7 @@ def _glob(self, pattern, case_sensitive, follow_symlinks):
             pattern_parts.append('')
         if pattern_parts[-1] == '**':
             # GH-70303: '**' only matches directories. Add trailing slash.
+            import warnings
             warnings.warn(
                 "Pattern ending '**' will match files and directories in a "
                 "future Python release. Add a trailing slash to match only "

Lib/test/test_pathlib/test_pathlib.py

@@ -213,6 +213,19 @@ def test_repr_roundtrips(self):
                 self.assertEqual(q, p)
                 self.assertEqual(repr(q), r)
 
+    def test_relative_to_several_args(self):
+        P = self.cls
+        p = P('a/b')
+        with self.assertWarns(DeprecationWarning):
+            p.relative_to('a', 'b')
+            p.relative_to('a', 'b', walk_up=True)
+
+    def test_is_relative_to_several_args(self):
+        P = self.cls
+        p = P('a/b')
+        with self.assertWarns(DeprecationWarning):
+            p.is_relative_to('a', 'b')
+
 
 class PurePosixPathTest(PurePathTest):
     cls = pathlib.PurePosixPath

Lib/test/test_pathlib/test_pathlib_abc.py

@@ -543,10 +543,6 @@ def test_relative_to_common(self):
         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.
-        with self.assertWarns(DeprecationWarning):
-            p.relative_to('a', 'b')
-            p.relative_to('a', 'b', walk_up=True)
         # Unrelated paths.
         self.assertRaises(ValueError, p.relative_to, P('c'))
         self.assertRaises(ValueError, p.relative_to, P('a/b/c'))
@@ -608,9 +604,6 @@ def test_is_relative_to_common(self):
         self.assertTrue(p.is_relative_to('a/'))
         self.assertTrue(p.is_relative_to(P('a/b')))
         self.assertTrue(p.is_relative_to('a/b'))
-        # With several args.
-        with self.assertWarns(DeprecationWarning):
-            p.is_relative_to('a', 'b')
         # Unrelated paths.
         self.assertFalse(p.is_relative_to(P('c')))
         self.assertFalse(p.is_relative_to(P('a/b/c')))