gh-149083: Use sentinel for os.path.{ALLOW_MISSING,ALL_BUT_LAST}

Author: JelleZijlstra

Doc/library/os.path.rst

@@ -482,13 +482,13 @@ the :mod:`glob` module.)
 
 .. data:: ALL_BUT_LAST
 
-   Special value used for the *strict* argument in :func:`realpath`.
+   :class:`sentinel` used for the *strict* argument in :func:`realpath`.
 
    .. versionadded:: 3.15
 
 .. data:: ALLOW_MISSING
 
-   Special value used for the *strict* argument in :func:`realpath`.
+   :class:`sentinel` used for the *strict* argument in :func:`realpath`.
 
    .. versionadded:: 3.15
 

Lib/genericpath.py

@@ -201,20 +201,8 @@ def _check_arg_types(funcname, *args):
         raise TypeError("Can't mix strings and bytes in path components") from None
 
 
-# Singletons with a true boolean value.
-
-@object.__new__
-class ALL_BUT_LAST:
-    """Special value for use in realpath()."""
-    def __repr__(self):
-        return 'os.path.ALL_BUT_LAST'
-    def __reduce__(self):
-        return self.__class__.__name__
-
-@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__
+# Sentinels.
+ALL_BUT_LAST = sentinel('ALL_BUT_LAST')
+"""Special value for use in realpath()."""
+ALLOW_MISSING = sentinel('ALLOW_MISSING')
+"""Special value for use in realpath()."""

Misc/NEWS.d/next/Library/2026-05-09-10-58-34.gh-issue-149083.I53mx4.rst

@@ -0,0 +1,2 @@
+:data:`os.path.ALLOW_MISSING` and :data:`os.path.ALL_BUT_LAST` are now
+instances of :class:`sentinel`.