GH-78079: Fix UNC device path root normalization in pathlib

Lib/pathlib.py

@@ -281,9 +281,14 @@ def _parse_parts(cls, parts):
         if altsep:
             path = path.replace(altsep, sep)
         drv, root, rel = cls._flavour.splitroot(path)
-        if drv.startswith(sep):
-            # pathlib assumes that UNC paths always have a root.
-            root = sep
+        if drv and not root and not drv.endswith(sep):
+            drv_parts = drv.split(sep)
+            if len(drv_parts) == 4 and drv_parts[2] not in '?.':
+                # e.g. //server/share
+                root = sep
+            elif len(drv_parts) == 6:
+                # e.g. //?/unc/server/share
+                root = sep
         unfiltered_parsed = [drv + root] + rel.split(sep)
         parsed = [sys.intern(x) for x in unfiltered_parsed if x and x != '.']
         return drv, root, parsed

Lib/test/test_ntpath.py

@@ -168,6 +168,7 @@ def test_splitroot(self):
 
         # gh-81790: support device namespace, including UNC drives.
         tester('ntpath.splitroot("//?/c:")', ("//?/c:", "", ""))
+        tester('ntpath.splitroot("//./c:")', ("//./c:", "", ""))
         tester('ntpath.splitroot("//?/c:/")', ("//?/c:", "/", ""))
         tester('ntpath.splitroot("//?/c:/dir")', ("//?/c:", "/", "dir"))
         tester('ntpath.splitroot("//?/UNC")', ("//?/UNC", "", ""))
@@ -178,8 +179,12 @@ def test_splitroot(self):
         tester('ntpath.splitroot("//?/VOLUME{00000000-0000-0000-0000-000000000000}/spam")',
                ('//?/VOLUME{00000000-0000-0000-0000-000000000000}', '/', 'spam'))
         tester('ntpath.splitroot("//?/BootPartition/")', ("//?/BootPartition", "/", ""))
+        tester('ntpath.splitroot("//./BootPartition/")', ("//./BootPartition", "/", ""))
+        tester('ntpath.splitroot("//./PhysicalDrive0")', ("//./PhysicalDrive0", "", ""))
+        tester('ntpath.splitroot("//./nul")', ("//./nul", "", ""))
 
         tester('ntpath.splitroot("\\\\?\\c:")', ("\\\\?\\c:", "", ""))
+        tester('ntpath.splitroot("\\\\.\\c:")', ("\\\\.\\c:", "", ""))
         tester('ntpath.splitroot("\\\\?\\c:\\")', ("\\\\?\\c:", "\\", ""))
         tester('ntpath.splitroot("\\\\?\\c:\\dir")', ("\\\\?\\c:", "\\", "dir"))
         tester('ntpath.splitroot("\\\\?\\UNC")', ("\\\\?\\UNC", "", ""))
@@ -192,6 +197,9 @@ def test_splitroot(self):
         tester('ntpath.splitroot("\\\\?\\VOLUME{00000000-0000-0000-0000-000000000000}\\spam")',
                ('\\\\?\\VOLUME{00000000-0000-0000-0000-000000000000}', '\\', 'spam'))
         tester('ntpath.splitroot("\\\\?\\BootPartition\\")', ("\\\\?\\BootPartition", "\\", ""))
+        tester('ntpath.splitroot("\\\\.\\BootPartition\\")', ("\\\\.\\BootPartition", "\\", ""))
+        tester('ntpath.splitroot("\\\\.\\PhysicalDrive0")', ("\\\\.\\PhysicalDrive0", "", ""))
+        tester('ntpath.splitroot("\\\\.\\nul")', ("\\\\.\\nul", "", ""))
 
         # gh-96290: support partial/invalid UNC drives
         tester('ntpath.splitroot("//")', ("//", "", ""))  # empty server & missing share

Lib/test/test_pathlib.py

@@ -95,6 +95,9 @@ def test_parse_parts(self):
         check(['c:/a'],                 ('c:', '\\', ['c:\\', 'a']))
         check(['/a'],                   ('', '\\', ['\\', 'a']))
         # UNC paths.
+        check(['//'],                   ('\\\\', '', ['\\\\']))
+        check(['//a'],                  ('\\\\a', '', ['\\\\a']))
+        check(['//a/'],                 ('\\\\a\\', '', ['\\\\a\\']))
         check(['//a/b'],                ('\\\\a\\b', '\\', ['\\\\a\\b\\']))
         check(['//a/b/'],               ('\\\\a\\b', '\\', ['\\\\a\\b\\']))
         check(['//a/b/c'],              ('\\\\a\\b', '\\', ['\\\\a\\b\\', 'c']))
@@ -108,12 +111,26 @@ def test_parse_parts(self):
         # UNC paths.
         check(['a', '//b/c//', 'd'],    ('\\\\b\\c', '\\', ['\\\\b\\c\\', 'd']))
         # Extended paths.
+        check(['//./c:'],               ('\\\\.\\c:', '', ['\\\\.\\c:']))
         check(['//?/c:/'],              ('\\\\?\\c:', '\\', ['\\\\?\\c:\\']))
         check(['//?/c:/a'],             ('\\\\?\\c:', '\\', ['\\\\?\\c:\\', 'a']))
         check(['//?/c:/a', '/b'],       ('\\\\?\\c:', '\\', ['\\\\?\\c:\\', 'b']))
         # Extended UNC paths (format is "\\?\UNC\server\share").
+        check(['//?'],                  ('\\\\?', '', ['\\\\?']))
+        check(['//?/'],                 ('\\\\?\\', '', ['\\\\?\\']))
+        check(['//?/UNC'],              ('\\\\?\\UNC', '', ['\\\\?\\UNC']))
+        check(['//?/UNC/'],             ('\\\\?\\UNC\\', '', ['\\\\?\\UNC\\']))
+        check(['//?/UNC/b'],            ('\\\\?\\UNC\\b', '', ['\\\\?\\UNC\\b']))
+        check(['//?/UNC/b/'],           ('\\\\?\\UNC\\b\\', '', ['\\\\?\\UNC\\b\\']))
         check(['//?/UNC/b/c'],          ('\\\\?\\UNC\\b\\c', '\\', ['\\\\?\\UNC\\b\\c\\']))
+        check(['//?/UNC/b/c/'],         ('\\\\?\\UNC\\b\\c', '\\', ['\\\\?\\UNC\\b\\c\\']))
         check(['//?/UNC/b/c/d'],        ('\\\\?\\UNC\\b\\c', '\\', ['\\\\?\\UNC\\b\\c\\', 'd']))
+        # UNC device paths
+        check(['//./BootPartition/'],   ('\\\\.\\BootPartition', '\\', ['\\\\.\\BootPartition\\']))
+        check(['//?/BootPartition/'],   ('\\\\?\\BootPartition', '\\', ['\\\\?\\BootPartition\\']))
+        check(['//./PhysicalDrive0'],   ('\\\\.\\PhysicalDrive0', '', ['\\\\.\\PhysicalDrive0']))
+        check(['//?/Volume{}/'],        ('\\\\?\\Volume{}', '\\', ['\\\\?\\Volume{}\\']))
+        check(['//./nul'],              ('\\\\.\\nul', '', ['\\\\.\\nul']))
         # Second part has a root but not drive.
         check(['a', '/b', 'c'],         ('', '\\', ['\\', 'b', 'c']))
         check(['Z:/a', '/b', 'c'],      ('Z:', '\\', ['Z:\\', 'b', 'c']))

Misc/NEWS.d/next/Library/2023-02-17-21-14-40.gh-issue-78079.z3Szr6.rst

@@ -0,0 +1,3 @@
+Fix incorrect normalization of UNC device path roots, and partial UNC share
+path roots, in :class:`pathlib.PurePath`. Pathlib no longer appends a
+trailing slash to such paths.