gh-89760: Fix os.path.realpath() for volume GUID paths on Windows

Lib/ntpath.py

@@ -666,6 +666,7 @@ def realpath(path, /, *, strict=False):
  prefix = b'\\\\?\\'
  unc_prefix = b'\\\\?\\UNC\\'
  new_unc_prefix = b'\\\\'
+ colon_sep = b':\\'
  cwd = os.getcwdb()
  # bpo-38081: Special case for realpath(b'nul')
  devnull = b'nul'
@@ -675,6 +676,7 @@ def realpath(path, /, *, strict=False):
  prefix = '\\\\?\\'
  unc_prefix = '\\\\?\\UNC\\'
  new_unc_prefix = '\\\\'
+ colon_sep = ':\\'
  cwd = os.getcwd()
  # bpo-38081: Special case for realpath('nul')
  devnull = 'nul'
@@ -718,25 +720,29 @@ def realpath(path, /, *, strict=False):
  # strip off that prefix unless it was already provided on the original
  # path.
  if not had_prefix and path.startswith(prefix):
- # For UNC paths, the prefix will actually be \\?\UNC\
- # Handle that case as well.
+ # For UNC drives, the path starts with \\?\UNC\.
  if path.startswith(unc_prefix):
  spath = new_unc_prefix + path[len(unc_prefix):]
- else:
+ # For drive-letter drives, the path starts with \\?\<letter>:\.
+ elif path.startswith(colon_sep, len(prefix) + 1):
  spath = path[len(prefix):]
- # Ensure that the non-prefixed path resolves to the same path
- try:
- if _getfinalpathname(spath) == path:
- path = spath
- except ValueError:
- # Unexpected, as an invalid path should not have gained a prefix
- # at any point, but we ignore this error just in case.
- pass
- except OSError as ex:
- # If the path does not exist and originally did not exist, then
- # strip the prefix anyway.
- if ex.winerror == initial_winerror:
- path = spath
+ # For all others, e.g. volume GUID paths, it cannot be stripped.
+ else:
+ spath = None
+ if spath is not None:
+ # Ensure that the non-prefixed path resolves to the same path
+ try:
+ if _getfinalpathname(spath) == path:
+ path = spath
+ except ValueError:
+ # Unexpected, as an invalid path should not have gained a
+ # prefix at any point, but we ignore this error just in case.
+ pass
+ except OSError as ex:
+ # If the path does not exist and originally did not exist,
+ # then strip the prefix anyway.
+ if ex.winerror == initial_winerror:
+ path = spath
  return path
 
 

Lib/test/test_ntpath.py

@@ -1535,6 +1535,33 @@ def test_isjunction(self):
  self.assertFalse(ntpath.isjunction('tmpdir'))
  self.assertPathEqual(ntpath.realpath('testjunc'), ntpath.realpath('tmpdir'))
 
+ @unittest.skipIf(sys.platform != 'win32', "Can only test junctions with creation on win32.")
+ def test_realpath_volume_guid_path(self):
+ # gh-89760: the \\?\ prefix cannot be stripped from a volume GUID path.
+ # Find a volume which is not mounted as a drive.
+ for volume in os.listvolumes():
+ if not os.listmounts(volume):
+ break
+ else:
+ raise unittest.SkipTest('no volume without a mount point')
+
+ with os_helper.temp_dir() as d:
+ with os_helper.change_cwd(d):
+ # _winapi.CreateJunction() adds the \\??\\ prefix to a path
+ # which already has a prefix.
+ try:
+ subprocess.run(['cmd', '/c', 'mklink', '/j',
+ 'testjunc', volume],
+ check=True, capture_output=True)
+ except (OSError, subprocess.CalledProcessError):
+ raise unittest.SkipTest('creating the test junction failed')
+
+ for path in 'testjunc', 'testjunc/spam', 'testjunc/spam/eggs':
+ with self.subTest(path=path):
+ realpath = ntpath.realpath(path)
+ self.assertStartsWith(realpath, '\\\\?\\Volume{')
+ self.assertTrue(ntpath.isabs(realpath), realpath)
+
  def test_isfile_invalid_paths(self):
  isfile = ntpath.isfile
  self.assertIs(isfile('/tmp\udfffabcds'), False)

Misc/NEWS.d/next/Library/2026-08-09-00-30-00.gh-issue-89760.rpvol.rst

@@ -0,0 +1,2 @@
+Fix :func:`os.path.realpath` on Windows: the ``\\?\`` prefix is no longer
+stripped from a volume GUID path, which made the result invalid.