gh-102475: Fix os.path.realpath() for names which look like a drive

Lib/ntpath.py

@@ -626,12 +626,23 @@ def _getfinalpathname_nonstrict(path, ignored_error=OSError):
  allowed_winerror = 1, 2, 3, 5, 21, 32, 50, 53, 65, 67, 87, 123, 161, 1005, 1920, 1921
 
  # Non-strict algorithm is to find as much of the target directory
- # as we can and join the rest.
+ # as we can and join the rest. join() is not used, because the tail
+ # can contain a colon and be mistaken for a drive (gh-102475).
+ if isinstance(path, bytes):
+ sep = b'\\'
+ else:
+ sep = '\\'
+
+ def join(path, tail):
+ if path[-1:] == sep or not tail:
+ return path + tail
+ return path + sep + tail
+
  tail = path[:0]
  while path:
  try:
  path = _getfinalpathname(path)
- return join(path, tail) if tail else path
+ return join(path, tail)
  except ignored_error as ex:
  if ex.winerror not in allowed_winerror:
  raise
@@ -642,7 +653,7 @@ def _getfinalpathname_nonstrict(path, ignored_error=OSError):
  new_path = _readlink_deep(path,
  ignored_error=ignored_error)
  if new_path != path:
- return join(new_path, tail) if tail else new_path
+ return join(new_path, tail)
  except ignored_error:
  # If we fail to readlink(), let's keep traversing
  pass
@@ -657,7 +668,7 @@ def _getfinalpathname_nonstrict(path, ignored_error=OSError):
  path, name = split(path)
  if path and not name:
  return path + tail
- tail = join(name, tail) if tail else name
+ tail = join(name, tail)
  return tail
 
  def realpath(path, /, *, strict=False):
@@ -666,7 +677,6 @@ def realpath(path, /, *, strict=False):
  prefix = b'\\\\?\\'
  unc_prefix = b'\\\\?\\UNC\\'
  new_unc_prefix = b'\\\\'
- cwd = os.getcwdb()
  # bpo-38081: Special case for realpath(b'nul')
  devnull = b'nul'
  if normcase(path) == devnull:
@@ -675,7 +685,6 @@ def realpath(path, /, *, strict=False):
  prefix = '\\\\?\\'
  unc_prefix = '\\\\?\\UNC\\'
  new_unc_prefix = '\\\\'
- cwd = os.getcwd()
  # bpo-38081: Special case for realpath('nul')
  devnull = 'nul'
  if normcase(path) == devnull:
@@ -692,7 +701,9 @@ def realpath(path, /, *, strict=False):
  ignored_error = OSError
 
  if not had_prefix and not isabs(path):
- path = join(cwd, path)
+ # abspath() is used instead of join(cwd, path), because the path
+ # can be relative to another drive (gh-102475).
+ path = abspath(path)
  try:
  path = _getfinalpathname(path)
  initial_winerror = 0

Lib/test/test_ntpath.py

@@ -1535,6 +1535,35 @@ 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 on win32.")
+ def test_realpath_drive_like_names(self):
+ # gh-102475: the unresolved tail is appended, not joined, so a name
+ # which looks like a drive does not reset the path.
+ drive = ntpath.splitroot(os.getcwd())[0]
+ for path, expected in [
+ ('C:/spam:eggs', 'C:\\spam:eggs'),
+ ('C:/nonexistent/spam:eggs', 'C:\\nonexistent\\spam:eggs'),
+ ('C:/spam:eggs/ham', 'C:\\spam:eggs\\ham'),
+ ('C:/nonexistent/spam:eggs/ham', 'C:\\nonexistent\\spam:eggs\\ham'),
+ ]:
+ with self.subTest(path=path):
+ self.assertEqual(ntpath.realpath(path), expected)
+ self.assertEqual(ntpath.realpath(os.fsencode(path)),
+ os.fsencode(expected))
+
+ @unittest.skipIf(sys.platform != 'win32', "Can only test on win32.")
+ def test_realpath_drive_relative(self):
+ # gh-102475: the working directory of a drive which does not exist
+ # is its root directory.
+ for drive in 'ABCDEFGHIJKLMNOPQRSTUVWXYZ':
+ if not ntpath.exists(drive + ':'):
+ break
+ else:
+ raise unittest.SkipTest('all drives exist')
+ self.assertEqual(ntpath.realpath(drive + ':spam'),
+ drive + ':\\spam')
+ self.assertEqual(ntpath.realpath(drive + ':'), drive + ':\\')
+
  def test_isfile_invalid_paths(self):
  isfile = ntpath.isfile
  self.assertIs(isfile('/tmp\udfffabcds'), False)

Misc/NEWS.d/next/Library/2026-08-09-01-30-00.gh-issue-102475.rpjoin.rst

@@ -0,0 +1,4 @@
+Fix :func:`os.path.realpath` on Windows: the unresolved part of the path is
+now appended, not joined, so a file name which looks like a drive (e.g.
+``spam:eggs``) no longer discards the resolved part. A path relative to
+another drive is now resolved against the root directory of that drive.