gh-135034: Switch to singleton

Doc/library/os.path.rst

@@ -424,7 +424,7 @@ the :mod:`glob` module.)
    In particular, :exc:`FileNotFoundError` is raised if *path* does not exist,
    or another :exc:`OSError` if it is otherwise inaccessible.
 
-   If *strict* is the string ``'allow_missing'``, errors other than
+   If *strict* is :py:data:`os.path.ALLOW_MISSING`, errors other than
    :exc:`FileNotFoundError` are re-raised (as with ``strict=True``).
    Thus, the returned path will not contain any symbolic links, but the named
    file and some of its parent directories may be missing.
@@ -447,7 +447,13 @@ the :mod:`glob` module.)
       The *strict* parameter was added.
 
    .. versionchanged:: next
-      The ``'allow_missing'`` value for *strict* parameter was added.
+      The :py:data:`~ntpath.ALLOW_MISSING` value for *strict* parameter was added.
+
+.. data:: ALLOW_MISSING
+
+   Special value used for the *strict* argument in :func:`realpath`.
+
+   .. versionadded:: next
 
 .. function:: relpath(path, start=os.curdir)
 

Doc/whatsnew/3.15.rst

@@ -116,7 +116,7 @@ os.path
 -------
 
 * The *strict* parameter to :func:`os.path.realpath` accepts a new value,
-  ``'allow_missing'``.
+  :data:`os.path.ALLOW_MISSING`.
   If used, errors other than :exc:`FileNotFoundError` will be re-raised;
   the resulting path can be missing but it will be free of symlinks.
   (Contributed by Petr Viktorin for :cve:`2025-4517`.)

Lib/genericpath.py

@@ -8,7 +8,7 @@
 
 __all__ = ['commonprefix', 'exists', 'getatime', 'getctime', 'getmtime',
            'getsize', 'isdevdrive', 'isdir', 'isfile', 'isjunction', 'islink',
-           'lexists', 'samefile', 'sameopenfile', 'samestat']
+           'lexists', 'samefile', 'sameopenfile', 'samestat', 'ALLOW_MISSING']
 
 
 # Does a path exist?
@@ -189,3 +189,12 @@ def _check_arg_types(funcname, *args):
                             f'os.PathLike object, not {s.__class__.__name__!r}') from None
     if hasstr and hasbytes:
         raise TypeError("Can't mix strings and bytes in path components") from None
+
+# A singleton with a true boolean value.
+@object.__new__
+class ALLOW_MISSING:
+    """Special value for use in realpath()."""
+    def __repr__(self):
+        return 'os.path.ALLOW_MISSING'
+    def __reduce__(self):
+        return self.__class__.__name__

Lib/ntpath.py

@@ -29,7 +29,7 @@
            "abspath","curdir","pardir","sep","pathsep","defpath","altsep",
            "extsep","devnull","realpath","supports_unicode_filenames","relpath",
            "samefile", "sameopenfile", "samestat", "commonpath", "isjunction",
-           "isdevdrive"]
+           "isdevdrive", "ALLOW_MISSING"]
 
 def _get_bothseps(path):
     if isinstance(path, bytes):
@@ -724,7 +724,7 @@ def realpath(path, *, strict=False):
                 return '\\\\.\\NUL'
         had_prefix = path.startswith(prefix)
 
-        if strict == 'allow_missing':
+        if strict is ALLOW_MISSING:
             ignored_error = FileNotFoundError
             strict = True
         elif strict:

Lib/posixpath.py

@@ -36,7 +36,7 @@
            "samefile","sameopenfile","samestat",
            "curdir","pardir","sep","pathsep","defpath","altsep","extsep",
            "devnull","realpath","supports_unicode_filenames","relpath",
-           "commonpath", "isjunction","isdevdrive"]
+           "commonpath", "isjunction","isdevdrive","ALLOW_MISSING"]
 
 
 def _get_sep(path):
@@ -402,7 +402,7 @@ def realpath(filename, *, strict=False):
         curdir = '.'
         pardir = '..'
         getcwd = os.getcwd
-    if strict == 'allow_missing':
+    if strict is ALLOW_MISSING:
         ignored_error = FileNotFoundError
         strict = True
     elif strict:

Lib/tarfile.py

@@ -781,7 +781,7 @@ def __init__(self, tarinfo, path):
 def _get_filtered_attrs(member, dest_path, for_data=True):
     new_attrs = {}
     name = member.name
-    dest_path = os.path.realpath(dest_path, strict='allow_missing')
+    dest_path = os.path.realpath(dest_path, strict=os.path.ALLOW_MISSING)
     # Strip leading / (tar's directory separator) from filenames.
     # Include os.sep (target OS directory separator) as well.
     if name.startswith(('/', os.sep)):
@@ -792,7 +792,7 @@ def _get_filtered_attrs(member, dest_path, for_data=True):
         raise AbsolutePathError(member)
     # Ensure we stay in the destination
     target_path = os.path.realpath(os.path.join(dest_path, name),
-                                   strict='allow_missing')
+                                   strict=os.path.ALLOW_MISSING)
     if os.path.commonpath([target_path, dest_path]) != dest_path:
         raise OutsideDestinationError(member, target_path)
     # Limit permissions (no high bits, and go-w)
@@ -840,7 +840,8 @@ def _get_filtered_attrs(member, dest_path, for_data=True):
             else:
                 target_path = os.path.join(dest_path,
                                            member.linkname)
-            target_path = os.path.realpath(target_path, strict='allow_missing')
+            target_path = os.path.realpath(target_path,
+                                           strict=os.path.ALLOW_MISSING)
             if os.path.commonpath([target_path, dest_path]) != dest_path:
                 raise LinkOutsideDestinationError(member, target_path)
     return new_attrs

Lib/test/test_ntpath.py

@@ -6,6 +6,7 @@
 import sys
 import unittest
 import warnings
+from ntpath import ALLOW_MISSING
 from test.support import TestFailed, cpython_only, os_helper
 from test.support.os_helper import FakePath
 from test import test_genericpath
@@ -505,15 +506,15 @@ def test_realpath_curdir_strict(self):
 
     def test_realpath_curdir_missing_ok(self):
         expected = ntpath.normpath(os.getcwd())
-        tester("ntpath.realpath('.', strict='allow_missing')",
+        tester("ntpath.realpath('.', strict=ALLOW_MISSING)",
                expected)
-        tester("ntpath.realpath('./.', strict='allow_missing')",
+        tester("ntpath.realpath('./.', strict=ALLOW_MISSING)",
                expected)
-        tester("ntpath.realpath('/'.join(['.'] * 100), strict='allow_missing')",
+        tester("ntpath.realpath('/'.join(['.'] * 100), strict=ALLOW_MISSING)",
                expected)
-        tester("ntpath.realpath('.\\.', strict='allow_missing')",
+        tester("ntpath.realpath('.\\.', strict=ALLOW_MISSING)",
                expected)
-        tester("ntpath.realpath('\\'.join(['.'] * 100), strict='allow_missing')",
+        tester("ntpath.realpath('\\'.join(['.'] * 100), strict=ALLOW_MISSING)",
                expected)
 
     def test_realpath_pardir(self):
@@ -542,20 +543,20 @@ def test_realpath_pardir_strict(self):
 
     def test_realpath_pardir_missing_ok(self):
         expected = ntpath.normpath(os.getcwd())
-        tester("ntpath.realpath('..', strict='allow_missing')",
+        tester("ntpath.realpath('..', strict=ALLOW_MISSING)",
                ntpath.dirname(expected))
-        tester("ntpath.realpath('../..', strict='allow_missing')",
+        tester("ntpath.realpath('../..', strict=ALLOW_MISSING)",
                ntpath.dirname(ntpath.dirname(expected)))
-        tester("ntpath.realpath('/'.join(['..'] * 50), strict='allow_missing')",
+        tester("ntpath.realpath('/'.join(['..'] * 50), strict=ALLOW_MISSING)",
                ntpath.splitdrive(expected)[0] + '\\')
-        tester("ntpath.realpath('..\\..', strict='allow_missing')",
+        tester("ntpath.realpath('..\\..', strict=ALLOW_MISSING)",
                ntpath.dirname(ntpath.dirname(expected)))
-        tester("ntpath.realpath('\\'.join(['..'] * 50), strict='allow_missing')",
+        tester("ntpath.realpath('\\'.join(['..'] * 50), strict=ALLOW_MISSING)",
                ntpath.splitdrive(expected)[0] + '\\')
 
     @os_helper.skip_unless_symlink
     @unittest.skipUnless(HAVE_GETFINALPATHNAME, 'need _getfinalpathname')
-    @_parameterize({}, {'strict': True}, {'strict': 'allow_missing'})
+    @_parameterize({}, {'strict': True}, {'strict': ALLOW_MISSING})
     def test_realpath_basic(self, kwargs):
         ABSTFN = ntpath.abspath(os_helper.TESTFN)
         open(ABSTFN, "wb").close()
@@ -603,38 +604,38 @@ def test_realpath_invalid_paths(self):
         self.assertEqual(realpath(path, strict=False), path)
         # gh-106242: Embedded nulls should raise OSError (not ValueError)
         self.assertRaises(OSError, realpath, path, strict=True)
-        self.assertRaises(OSError, realpath, path, strict='allow_missing')
+        self.assertRaises(OSError, realpath, path, strict=ALLOW_MISSING)
         path = ABSTFNb + b'\x00'
         self.assertEqual(realpath(path, strict=False), path)
         self.assertRaises(OSError, realpath, path, strict=True)
-        self.assertRaises(OSError, realpath, path, strict='allow_missing')
+        self.assertRaises(OSError, realpath, path, strict=ALLOW_MISSING)
         path = ABSTFN + '\\nonexistent\\x\x00'
         self.assertEqual(realpath(path, strict=False), path)
         self.assertRaises(OSError, realpath, path, strict=True)
-        self.assertRaises(OSError, realpath, path, strict='allow_missing')
+        self.assertRaises(OSError, realpath, path, strict=ALLOW_MISSING)
         path = ABSTFNb + b'\\nonexistent\\x\x00'
         self.assertEqual(realpath(path, strict=False), path)
         self.assertRaises(OSError, realpath, path, strict=True)
-        self.assertRaises(OSError, realpath, path, strict='allow_missing')
+        self.assertRaises(OSError, realpath, path, strict=ALLOW_MISSING)
         path = ABSTFN + '\x00\\..'
         self.assertEqual(realpath(path, strict=False), os.getcwd())
         self.assertEqual(realpath(path, strict=True), os.getcwd())
-        self.assertEqual(realpath(path, strict='allow_missing'), os.getcwd())
+        self.assertEqual(realpath(path, strict=ALLOW_MISSING), os.getcwd())
         path = ABSTFNb + b'\x00\\..'
         self.assertEqual(realpath(path, strict=False), os.getcwdb())
         self.assertEqual(realpath(path, strict=True), os.getcwdb())
-        self.assertEqual(realpath(path, strict='allow_missing'), os.getcwdb())
+        self.assertEqual(realpath(path, strict=ALLOW_MISSING), os.getcwdb())
         path = ABSTFN + '\\nonexistent\\x\x00\\..'
         self.assertEqual(realpath(path, strict=False), ABSTFN + '\\nonexistent')
         self.assertRaises(OSError, realpath, path, strict=True)
-        self.assertEqual(realpath(path, strict='allow_missing'), ABSTFN + '\\nonexistent')
+        self.assertEqual(realpath(path, strict=ALLOW_MISSING), ABSTFN + '\\nonexistent')
         path = ABSTFNb + b'\\nonexistent\\x\x00\\..'
         self.assertEqual(realpath(path, strict=False), ABSTFNb + b'\\nonexistent')
         self.assertRaises(OSError, realpath, path, strict=True)
-        self.assertEqual(realpath(path, strict='allow_missing'), ABSTFNb + b'\\nonexistent')
+        self.assertEqual(realpath(path, strict=ALLOW_MISSING), ABSTFNb + b'\\nonexistent')
 
     @unittest.skipUnless(HAVE_GETFINALPATHNAME, 'need _getfinalpathname')
-    @_parameterize({}, {'strict': True}, {'strict': 'allow_missing'})
+    @_parameterize({}, {'strict': True}, {'strict': ALLOW_MISSING})
     def test_realpath_invalid_unicode_paths(self, kwargs):
         realpath = ntpath.realpath
         ABSTFN = ntpath.abspath(os_helper.TESTFN)
@@ -654,7 +655,7 @@ def test_realpath_invalid_unicode_paths(self, kwargs):
 
     @os_helper.skip_unless_symlink
     @unittest.skipUnless(HAVE_GETFINALPATHNAME, 'need _getfinalpathname')
-    @_parameterize({}, {'strict': True}, {'strict': 'allow_missing'})
+    @_parameterize({}, {'strict': True}, {'strict': ALLOW_MISSING})
     def test_realpath_relative(self, kwargs):
         ABSTFN = ntpath.abspath(os_helper.TESTFN)
         open(ABSTFN, "wb").close()
@@ -815,7 +816,7 @@ def test_realpath_symlink_loops_strict(self):
     @os_helper.skip_unless_symlink
     @unittest.skipUnless(HAVE_GETFINALPATHNAME, 'need _getfinalpathname')
     def test_realpath_symlink_loops_raise(self):
-        # Symlink loops raise OSError in 'allow_missing' mode
+        # Symlink loops raise OSError in ALLOW_MISSING mode
         ABSTFN = ntpath.abspath(os_helper.TESTFN)
         self.addCleanup(os_helper.unlink, ABSTFN)
         self.addCleanup(os_helper.unlink, ABSTFN + "1")
@@ -826,16 +827,16 @@ def test_realpath_symlink_loops_raise(self):
         self.addCleanup(os_helper.unlink, ABSTFN + "x")
 
         os.symlink(ABSTFN, ABSTFN)
-        self.assertRaises(OSError, ntpath.realpath, ABSTFN, strict='allow_missing')
+        self.assertRaises(OSError, ntpath.realpath, ABSTFN, strict=ALLOW_MISSING)
 
         os.symlink(ABSTFN + "1", ABSTFN + "2")
         os.symlink(ABSTFN + "2", ABSTFN + "1")
         self.assertRaises(OSError, ntpath.realpath, ABSTFN + "1",
-                            strict='allow_missing')
+                            strict=ALLOW_MISSING)
         self.assertRaises(OSError, ntpath.realpath, ABSTFN + "2",
-                            strict='allow_missing')
+                            strict=ALLOW_MISSING)
         self.assertRaises(OSError, ntpath.realpath, ABSTFN + "1\\x",
-                            strict='allow_missing')
+                            strict=ALLOW_MISSING)
 
         # Windows eliminates '..' components before resolving links;
         # realpath is not expected to raise if this removes the loop.
@@ -851,24 +852,24 @@ def test_realpath_symlink_loops_raise(self):
         self.assertRaises(
             OSError, ntpath.realpath,
             ABSTFN + "1\\..\\" + ntpath.basename(ABSTFN) + "1",
-            strict='allow_missing')
+            strict=ALLOW_MISSING)
 
         os.symlink(ntpath.basename(ABSTFN) + "a\\b", ABSTFN + "a")
         self.assertRaises(OSError, ntpath.realpath, ABSTFN + "a",
-                            strict='allow_missing')
+                            strict=ALLOW_MISSING)
 
         os.symlink("..\\" + ntpath.basename(ntpath.dirname(ABSTFN))
                 + "\\" + ntpath.basename(ABSTFN) + "c", ABSTFN + "c")
         self.assertRaises(OSError, ntpath.realpath, ABSTFN + "c",
-                            strict='allow_missing')
+                            strict=ALLOW_MISSING)
 
         # Test using relative path as well.
         self.assertRaises(OSError, ntpath.realpath, ntpath.basename(ABSTFN),
-                            strict='allow_missing')
+                            strict=ALLOW_MISSING)
 
     @os_helper.skip_unless_symlink
     @unittest.skipUnless(HAVE_GETFINALPATHNAME, 'need _getfinalpathname')
-    @_parameterize({}, {'strict': True}, {'strict': 'allow_missing'})
+    @_parameterize({}, {'strict': True}, {'strict': ALLOW_MISSING})
     def test_realpath_symlink_prefix(self, kwargs):
         ABSTFN = ntpath.abspath(os_helper.TESTFN)
         self.addCleanup(os_helper.unlink, ABSTFN + "3")
@@ -906,7 +907,7 @@ def test_realpath_nul(self):
         tester("ntpath.realpath('NUL')", r'\\.\NUL')
         tester("ntpath.realpath('NUL', strict=False)", r'\\.\NUL')
         tester("ntpath.realpath('NUL', strict=True)", r'\\.\NUL')
-        tester("ntpath.realpath('NUL', strict='allow_missing')", r'\\.\NUL')
+        tester("ntpath.realpath('NUL', strict=ALLOW_MISSING)", r'\\.\NUL')
 
     @unittest.skipUnless(HAVE_GETFINALPATHNAME, 'need _getfinalpathname')
     @unittest.skipUnless(HAVE_GETSHORTPATHNAME, 'need _getshortpathname')
@@ -930,7 +931,7 @@ def test_realpath_cwd(self):
 
         self.assertPathEqual(test_file_long, ntpath.realpath(test_file_short))
 
-        for kwargs in {}, {'strict': True}, {'strict': 'allow_missing'}:
+        for kwargs in {}, {'strict': True}, {'strict': ALLOW_MISSING}:
             with self.subTest(**kwargs):
                 with os_helper.change_cwd(test_dir_long):
                     self.assertPathEqual(