gh-149083: Convert some private and simple sentinels to PEP 661

Lib/_collections_abc.py

@@ -925,7 +925,7 @@ def __setitem__(self, key, value):
     def __delitem__(self, key):
         raise KeyError
 
-    __marker = object()
+    __marker = sentinel("__marker")
 
     def pop(self, key, default=__marker):
         '''D.pop(k[,d]) -> v, remove specified key and return the corresponding value.

Lib/_pydatetime.py

@@ -2447,7 +2447,7 @@ class timezone(tzinfo):
     __slots__ = '_offset', '_name'
 
     # Sentinel value to disallow None
-    _Omitted = object()
+    _Omitted = sentinel("_Omitted")
     def __new__(cls, offset, name=_Omitted):
         if not isinstance(offset, timedelta):
             raise TypeError("offset must be a timedelta")

Lib/configparser.py

@@ -372,11 +372,6 @@ def __init__(self):
         Error.__init__(self, "Support for UNNAMED_SECTION is disabled.")
 
 
-class _UnnamedSection:
-
-    def __repr__(self):
-        return "<UNNAMED_SECTION>"
-
 class InvalidWriteError(Error):
     """Raised when attempting to write data that the parser would read back differently.
     ex: writing a key which begins with the section header pattern would read back as a
@@ -386,13 +381,13 @@ def __init__(self, msg=''):
         Error.__init__(self, msg)
 
 
-UNNAMED_SECTION = _UnnamedSection()
+UNNAMED_SECTION = sentinel("<UNNAMED_SECTION>")
 
 
 # Used in parser getters to indicate the default behaviour when a specific
 # option is not found it to raise an exception. Created to enable `None` as
 # a valid fallback value.
-_UNSET = object()
+_UNSET = sentinel("_UNSET")
 
 
 class Interpolation:

Lib/dataclasses.py

@@ -173,10 +173,7 @@ class FrozenInstanceError(AttributeError): pass
 # A sentinel object for default values to signal that a default
 # factory will be used.  This is given a nice repr() which will appear
 # in the function signature of dataclasses' constructors.
-class _HAS_DEFAULT_FACTORY_CLASS:
-    def __repr__(self):
-        return '<factory>'
-_HAS_DEFAULT_FACTORY = _HAS_DEFAULT_FACTORY_CLASS()
+_HAS_DEFAULT_FACTORY = sentinel('<factory>')
 
 # A sentinel object to detect if a parameter is supplied or not.  Use
 # a class to give it a better repr.

Lib/functools.py

@@ -1132,7 +1132,7 @@ def register(self):
 ### cached_property() - property result cached as instance attribute
 ################################################################################
 
-_NOT_FOUND = object()
+_NOT_FOUND = sentinel("_NOT_FOUND")
 
 class cached_property:
     def __init__(self, func):

Lib/inspect.py

@@ -1691,7 +1691,7 @@ def trace(context=1):
 
 # ------------------------------------------------ static version of getattr
 
-_sentinel = object()
+_sentinel = sentinel("_sentinel")
 _static_getmro = type.__dict__['__mro__'].__get__
 _get_dunder_dict_of_class = type.__dict__["__dict__"].__get__
 

Lib/sched.py

@@ -46,7 +46,7 @@
 Event.kwargs.__doc__ = ('''kwargs is a dictionary holding the keyword
 arguments for the action.''')
 
-_sentinel = object()
+_sentinel = sentinel("_sentinel")
 
 class scheduler:
 

Lib/traceback.py

@@ -106,11 +106,7 @@ def extract_tb(tb, limit=None):
     "another exception occurred:\n\n")
 
 
-class _Sentinel:
-    def __repr__(self):
-        return "<implicit>"
-
-_sentinel = _Sentinel()
+_sentinel = sentinel("<implicit>")
 
 def _parse_value_tb(exc, value, tb):
     if (value is _sentinel) != (tb is _sentinel):

Misc/NEWS.d/next/Library/2026-04-27-19-47-24.gh-issue-149083.xa_4JR.rst

@@ -0,0 +1,2 @@
+Various private sentinels in the standard library are now instances of
+:class:`sentinel`.