Support for pickling sentinel objects as singletons

doc/index.rst

@@ -1071,18 +1071,17 @@ 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.
+   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)
@@ -1095,6 +1094,15 @@ Sentinel objects
 
       See :pep:`661`
 
+   .. versionchanged:: 4.16.0
+
+      Now supports pickle and will be reduced as a singleton.
+      Renamed from `Sentinel` to `sentinel`, `Sentinel` is deprecated.
+      Automatic `repr` string no longer has angle brackets.
+      `repr` parameter was deprecated.
+      `name` as a keyword is deprecated.
+      Subclasssing and attribute assignment are deprecated.
+
 
 Pure aliases
 ~~~~~~~~~~~~

src/test_typing_extensions.py

@@ -102,6 +102,7 @@
     reveal_type,
     runtime,
     runtime_checkable,
+    sentinel,
     type_repr,
 )
 
@@ -9541,42 +9542,71 @@ 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):
+        with self.assertWarnsRegex(DeprecationWarning, r"'repr' is deprecated and must be removed"):
+            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_()
+
+    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
+        with self.assertRaisesRegex(TypeError, r"First parameter 'name' is required"):
             sentinel()
 
-    def test_sentinel_not_picklable(self):
-        sentinel = Sentinel('sentinel')
-        with self.assertRaisesRegex(
-            TypeError,
-            "Cannot pickle 'Sentinel' object"
-        ):
-            pickle.dumps(sentinel)
+        with self.assertWarnsRegex(DeprecationWarning, r"'name' is positional-only and must not be a keyword parameter"):
+            my_sentinel = Sentinel(name="my_sentinel")
+        with self.assertWarnsRegex(DeprecationWarning, r"Setting attributes on sentinel is deprecated"):
+            my_sentinel.foo = "bar"
+
 
 def load_tests(loader, tests, pattern):
     import doctest

src/typing_extensions.py

@@ -91,6 +91,7 @@
     'overload',
     'override',
     'Protocol',
+    'sentinel',
     'Sentinel',
     'reveal_type',
     'runtime',
@@ -159,22 +160,75 @@
 # Added with bpo-45166 to 3.10.1+ and some 3.9 versions
 _FORWARD_REF_HAS_CLASS = "__forward_is_class__" in typing.ForwardRef.__slots__
 
-class Sentinel:
+
+def _caller(depth=1, default='__main__'):
+    try:
+        return sys._getframemodulename(depth + 1) or default
+    except AttributeError:  # For platforms without _getframemodulename()
+        pass
+    try:
+        return sys._getframe(depth + 1).f_globals.get('__name__', default)
+    except (AttributeError, ValueError):  # For platforms without _getframe()
+        pass
+    return None
+
+
+# Placeholder for sentinel methods, because sentinels can not have their own sentinels
+_sentinel_placeholder = object()
+
+
+class sentinel:
     """Create a unique sentinel object.
 
     *name* should be the name of the variable to which the return value shall be assigned.
-
-    *repr*, if supplied, will be used for the repr of the sentinel object.
-    If not provided, "<name>" will be used.
     """
 
     def __init__(
         self,
-        name: str,
+        __name: str = _sentinel_placeholder,
+        /,
         repr: typing.Optional[str] = None,
-    ):
-        self._name = name
-        self._repr = repr if repr is not None else f'<{name}>'
+        *,
+        name: str = _sentinel_placeholder,
+    ) -> None:
+        if name is not _sentinel_placeholder:
+            warnings.warn(
+                "'name' is positional-only and must not be a keyword parameter",
+                DeprecationWarning,
+                stacklevel=2,
+            )
+            __name = name
+        if __name is _sentinel_placeholder:
+            raise TypeError("First parameter 'name' is required")
+        if repr is not None:
+            warnings.warn(
+                "'repr' is deprecated and must be removed",
+                DeprecationWarning,
+                stacklevel=2,
+            )
+
+        self.__name__ = __name
+        self._repr = repr if repr is not None else __name
+
+        # For pickling as a singleton:
+        self.__module__ = _caller()
+
+    def __init_subclass__(cls):
+        warnings.warn(
+            "Subclassing sentinel is forbidden by PEP 661",
+            DeprecationWarning,
+            stacklevel=2,
+        )
+        super().__init_subclass__()
+
+    def __setattr__(self, attr: str, value: object) -> None:
+        if attr not in {"__name__", "_repr", "__module__"}:
+            warnings.warn(
+                "Setting attributes on sentinel is deprecated",
+                DeprecationWarning,
+                stacklevel=2,
+            )
+        super().__setattr__(attr, value)
 
     def __repr__(self):
         return self._repr
@@ -193,11 +247,14 @@ def __or__(self, other):
         def __ror__(self, other):
             return typing.Union[other, self]
 
-    def __getstate__(self):
-        raise TypeError(f"Cannot pickle {type(self).__name__!r} object")
+    def __reduce__(self) -> str:
+        """Reduce this sentinel to a singleton."""
+        return self.__name__  # Module is taken from the __module__ attribute
+
+Sentinel = sentinel
 
+_marker = sentinel("sentinel")
 
-_marker = Sentinel("sentinel")
 
 # The functions below are modified copies of typing internal helpers.
 # They are needed by _ProtocolMeta and they provide support for PEP 646.
@@ -638,18 +695,6 @@ def _get_protocol_attrs(cls):
     return attrs
 
 
-def _caller(depth=1, default='__main__'):
-    try:
-        return sys._getframemodulename(depth + 1) or default
-    except AttributeError:  # For platforms without _getframemodulename()
-        pass
-    try:
-        return sys._getframe(depth + 1).f_globals.get('__name__', default)
-    except (AttributeError, ValueError):  # For platforms without _getframe()
-        pass
-    return None
-
-
 # `__match_args__` attribute was removed from protocol members in 3.13,
 # we want to backport this change to older Python versions.
 # Breakpoint: https://github.com/python/cpython/pull/110683