Support for pickling sentinel objects as singletons

doc/index.rst

@@ -1071,18 +1071,20 @@ Capsule objects
 Sentinel objects
 ~~~~~~~~~~~~~~~~
 
-.. class:: Sentinel(name, repr=None)
+.. class:: sentinel(name, /)
 
    A type used to define sentinel values. The *name* argument should be the
    name of the variable to which the return value shall be assigned.
 
-   If *repr* is provided, it will be used for the :meth:`~object.__repr__`
-   of the sentinel object. If not provided, ``"<name>"`` will be used.
+   A sentinel is bound to the module it is created within,
+   sentinels are not equal to similar named sentinels from other modules.
+
+   Assigning attributes to a sentinel including `__weakref__` is forbidden.
 
    Example::
 
-      >>> from typing_extensions import Sentinel, assert_type
-      >>> MISSING = Sentinel('MISSING')
+      >>> from typing_extensions import sentinel, assert_type
+      >>> MISSING = sentinel('MISSING')
       >>> def func(arg: int | MISSING = MISSING) -> None:
       ...     if arg is MISSING:
       ...         assert_type(arg, MISSING)
@@ -1091,6 +1093,13 @@ Sentinel objects
       ...
       >>> func(MISSING)
 
+   Sentinels defined inside a class scope should use a :term:`qualified name`.
+
+   Example::
+
+      >>> class MyClass:
+      ...     MISSING = sentinel('MyClass.MISSING')
+
    .. versionadded:: 4.14.0
 
       See :pep:`661`

src/test_typing_extensions.py

@@ -102,6 +102,7 @@
     reveal_type,
     runtime,
     runtime_checkable,
+    sentinel,
     type_repr,
 )
 
@@ -9541,42 +9542,67 @@ def test_invalid_special_forms(self):
 
 
 class TestSentinels(BaseTestCase):
+    SENTINEL = sentinel("TestSentinels.SENTINEL")
+
     def test_sentinel_no_repr(self):
-        sentinel_no_repr = Sentinel('sentinel_no_repr')
+        sentinel_no_repr = sentinel('sentinel_no_repr')
 
-        self.assertEqual(sentinel_no_repr._name, 'sentinel_no_repr')
-        self.assertEqual(repr(sentinel_no_repr), '<sentinel_no_repr>')
+        self.assertEqual(sentinel_no_repr.__name__, 'sentinel_no_repr')
+        self.assertEqual(repr(sentinel_no_repr), 'sentinel_no_repr')
 
-    def test_sentinel_explicit_repr(self):
-        sentinel_explicit_repr = Sentinel('sentinel_explicit_repr', repr='explicit_repr')
+    def test_sentinel_deprecated_explicit_repr(self):
+        sentinel_explicit_repr = sentinel('sentinel_explicit_repr', repr='explicit_repr')
 
         self.assertEqual(repr(sentinel_explicit_repr), 'explicit_repr')
 
     @skipIf(sys.version_info < (3, 10), reason='New unions not available in 3.9')
     def test_sentinel_type_expression_union(self):
-        sentinel = Sentinel('sentinel')
+        sentinel_type = sentinel('sentinel')
 
-        def func1(a: int | sentinel = sentinel): pass
-        def func2(a: sentinel | int = sentinel): pass
+        def func1(a: int | sentinel_type = sentinel_type): pass
+        def func2(a: sentinel_type | int = sentinel_type): pass
 
-        self.assertEqual(func1.__annotations__['a'], Union[int, sentinel])
-        self.assertEqual(func2.__annotations__['a'], Union[sentinel, int])
+        self.assertEqual(func1.__annotations__['a'], Union[int, sentinel_type])
+        self.assertEqual(func2.__annotations__['a'], Union[sentinel_type, int])
 
     def test_sentinel_not_callable(self):
-        sentinel = Sentinel('sentinel')
+        sentinel_ = sentinel('sentinel')
         with self.assertRaisesRegex(
             TypeError,
-            "'Sentinel' object is not callable"
+            "'sentinel' object is not callable"
         ):
-            sentinel()
+            sentinel_()
+
+    def test_sentinel_copy_identity(self):
+        self.assertIs(self.SENTINEL, copy.copy(self.SENTINEL))
+        self.assertIs(self.SENTINEL, copy.deepcopy(self.SENTINEL))
+
+        anonymous_sentinel = sentinel("anonymous_sentinel")
+        self.assertIs(anonymous_sentinel, copy.copy(anonymous_sentinel))
+        self.assertIs(anonymous_sentinel, copy.deepcopy(anonymous_sentinel))
+
+    def test_sentinel_picklable_qualified(self):
+        for proto in range(pickle.HIGHEST_PROTOCOL + 1):
+            self.assertIs(self.SENTINEL, pickle.loads(pickle.dumps(self.SENTINEL, protocol=proto)))
+
+    def test_sentinel_picklable_anonymous(self):
+        anonymous_sentinel = sentinel("anonymous_sentinel")  # Anonymous sentinel can not be pickled
+        for proto in range(pickle.HIGHEST_PROTOCOL + 1):
+            with self.assertRaisesRegex(
+                pickle.PicklingError,
+                r"attribute lookup anonymous_sentinel on \w+ failed|not found as \w+.anonymous_sentinel"
+            ):
+                self.assertIs(anonymous_sentinel, pickle.loads(pickle.dumps(anonymous_sentinel, protocol=proto)))
+
+    def test_sentinel_deprecated(self):
+        with self.assertWarnsRegex(DeprecationWarning, r"Subclassing sentinel is forbidden by PEP 661"):
+            class SentinelSubclass(Sentinel):
+                pass
+
+        my_sentinel = Sentinel(name="my_sentinel")
+        with self.assertWarnsRegex(DeprecationWarning, r"Setting attributes on sentinel is deprecated"):
+            my_sentinel.foo = "bar"
 
-    def test_sentinel_not_picklable(self):
-        sentinel = Sentinel('sentinel')
-        with self.assertRaisesRegex(
-            TypeError,
-            "Cannot pickle 'Sentinel' object"
-        ):
-            pickle.dumps(sentinel)
 
 def load_tests(loader, tests, pattern):
     import doctest