GH-125069: Fix inconsistent joining in `WindowsPath(PosixPath(...))` (#125156)

`PurePath.__init__()` incorrectly uses the `_raw_paths` of a given
`PurePath` object with a different flavour, even though the procedure to
join path segments can differ between flavours.

This change makes the `_raw_paths`-enabled deferred joining apply _only_
when the path flavours match.

Co-authored-by: Bénédikt Tran <10796600+picnixz@users.noreply.github.com>

Lib/pathlib/_local.py

@@ -119,9 +119,9 @@ class PurePath(PurePathBase):
         paths = []
         for arg in args:
             if isinstance(arg, PurePath):
-                if arg.parser is ntpath and self.parser is posixpath:
+                if arg.parser is not self.parser:
                     # GH-103631: Convert separators for backwards compatibility.
-                    paths.extend(path.replace('\\', '/') for path in arg._raw_paths)
+                    paths.append(arg.as_posix())
                 else:
                     paths.extend(arg._raw_paths)
             else:

Lib/test/test_pathlib/test_pathlib.py

@@ -131,6 +131,15 @@ class PurePathTest(test_pathlib_abc.DummyPurePathTest):
         self.assertEqual(P(P('a'), P('b'), P('c')), P(FakePath("a/b/c")))
         self.assertEqual(P(P('./a:b')), P('./a:b'))
 
+    @needs_windows
+    def test_constructor_nested_foreign_flavour(self):
+        # See GH-125069.
+        p1 = pathlib.PurePosixPath('b/c:\\d')
+        p2 = pathlib.PurePosixPath('b/', 'c:\\d')
+        self.assertEqual(p1, p2)
+        self.assertEqual(self.cls(p1), self.cls('b/c:/d'))
+        self.assertEqual(self.cls(p2), self.cls('b/c:/d'))
+
     def _check_parse_path(self, raw_path, *expected):
         sep = self.parser.sep
         actual = self.cls._parse_path(raw_path.replace('/', sep))

Misc/NEWS.d/next/Library/2024-10-08-21-17-16.gh-issue-125069.0RP0Mx.rst

@@ -0,0 +1,4 @@
+Fix an issue where providing a :class:`pathlib.PurePath` object as an
+initializer argument to a second :class:`~pathlib.PurePath` object with a
+different :attr:`~pathlib.PurePath.parser` resulted in arguments to the
+former object's initializer being joined by the latter object's parser.