Simplify Placeholder with the new sentinel() class

Author: rhettinger

Lib/functools.py

@@ -275,29 +275,7 @@ def reduce(function, sequence, /, initial=_initial_missing):
 ################################################################################
 
 
-class _PlaceholderType:
-    """The type of the Placeholder singleton.
-
-    Used as a placeholder for partial arguments.
-    """
-    __instance = None
-    __slots__ = ()
-
-    def __init_subclass__(cls, *args, **kwargs):
-        raise TypeError(f"type '{cls.__name__}' is not an acceptable base type")
-
-    def __new__(cls):
-        if cls.__instance is None:
-            cls.__instance = object.__new__(cls)
-        return cls.__instance
-
-    def __repr__(self):
-        return 'Placeholder'
-
-    def __reduce__(self):
-        return 'Placeholder'
-
-Placeholder = _PlaceholderType()
+Placeholder = sentinel('Placeholder') # _PlaceholderType()
 
 def _partial_prepare_merger(args):
     if not args:

Lib/test/test_functools.py

@@ -279,13 +279,6 @@ def test_placeholders_kw_restriction(self):
         self.assertEqual(len(actual_kwds), 1)
         self.assertIs(actual_kwds['a'], ALWAYS_EQ)
 
-    def test_construct_placeholder_singleton(self):
-        PH = self.module.Placeholder
-        tp = type(PH)
-        self.assertIs(tp(), PH)
-        self.assertRaises(TypeError, tp, 1, 2)
-        self.assertRaises(TypeError, tp, a=1, b=2)
-
     def test_repr(self):
         args = (object(), object())
         args_repr = ', '.join(repr(a) for a in args)