Add PEP 673 Self type (#933)

typing_extensions/CHANGELOG

@@ -8,6 +8,10 @@ Dropped support for Python versions 3.5 and older.
 Simplified backports for Python 3.6.0 and newer.
 Patch by Adam Turner (@AA-Turner).
 
+## Added in version 4.0.0
+
+- Runtime support for PEP 673 and `typing_extensions.Self`.
+
 ## Removed in version 4.0.0
 
 The following non-exported but non-private names have been removed as they are

typing_extensions/src_py3/test_typing_extensions.py

@@ -16,7 +16,7 @@
 from typing import Generic, NamedTuple
 from typing import no_type_check
 import typing_extensions
-from typing_extensions import NoReturn, ClassVar, Final, IntVar, Literal, Type, NewType, TypedDict
+from typing_extensions import NoReturn, ClassVar, Final, IntVar, Literal, Type, NewType, TypedDict, Self
 from typing_extensions import TypeAlias, ParamSpec, Concatenate, ParamSpecArgs, ParamSpecKwargs, TypeGuard
 from typing_extensions import Awaitable, AsyncIterator, AsyncContextManager
 from typing_extensions import Protocol, runtime, runtime_checkable, Annotated, overload
@@ -2075,6 +2075,42 @@ def test_no_isinstance(self):
             issubclass(int, TypeGuard)
 
 
+
+class SelfTests(BaseTestCase):
+    def test_basics(self):
+        class Foo:
+            def bar(self) -> Self: ...
+
+        self.assertEqual(gth(Foo.bar), {'return': Self})
+
+    def test_repr(self):
+        if hasattr(typing, 'Self'):
+            mod_name = 'typing'
+        else:
+            mod_name = 'typing_extensions'
+        self.assertEqual(repr(Self), '{}.Self'.format(mod_name))
+
+    def test_cannot_subscript(self):
+        with self.assertRaises(TypeError):
+            Self[int]
+
+    def test_cannot_subclass(self):
+        with self.assertRaises(TypeError):
+            class C(type(Self)):
+                pass
+
+    def test_cannot_init(self):
+        with self.assertRaises(TypeError):
+            Self()
+        with self.assertRaises(TypeError):
+            type(Self)()
+
+    def test_no_isinstance(self):
+        with self.assertRaises(TypeError):
+            isinstance(1, Self)
+        with self.assertRaises(TypeError):
+            issubclass(int, Self)
+
 class AllTests(BaseTestCase):
 
     def test_typing_extensions_includes_standard(self):

typing_extensions/src_py3/typing_extensions.py

@@ -45,6 +45,7 @@ def _check_generic(cls, parameters):
     'Concatenate',
     'Final',
     'ParamSpec',
+    'Self',
     'Type',
 
     # ABCs (from collections.abc).
@@ -2046,3 +2047,74 @@ def __eq__(self, other):
             return self is other
 
     TypeGuard = _TypeGuard(_root=True)
+
+
+if hasattr(typing, "Self"):
+    Self = typing.Self
+
+elif sys.version_info[:2] >= (3, 9):
+    class _SelfForm(typing._SpecialForm, _root=True):
+        def __repr__(self):
+            return 'typing_extensions.' + self._name
+
+    @_SelfForm
+    def Self(self, params):
+        """Used to spell the type of "self" in classes.
+
+        Example::
+
+          from typing import Self
+
+          class ReturnsSelf:
+              def parse(self, data: bytes) -> Self:
+                  ...
+                  return self
+
+        """
+
+        raise TypeError(f"{self} is not subscriptable")
+
+elif sys.version_info[:2] >= (3, 7):
+    class _SelfForm(typing._SpecialForm, _root=True):
+        def __repr__(self):
+            return 'typing_extensions.' + self._name
+
+    Self = _SelfForm(
+        "Self",
+        doc="""Used to spell the type of "self" in classes.
+
+        Example::
+
+          from typing import Self
+
+          class ReturnsSelf:
+              def parse(self, data: bytes) -> Self:
+                  ...
+                  return self
+
+        """
+    )
+else:
+    class _Self(typing._FinalTypingBase, _root=True):
+        """Used to spell the type of "self" in classes.
+
+        Example::
+
+          from typing import Self
+
+          class ReturnsSelf:
+              def parse(self, data: bytes) -> Self:
+                  ...
+                  return self
+
+        """
+
+        __slots__ = ()
+
+        def __instancecheck__(self, obj):
+            raise TypeError(f"{self} cannot be used with isinstance().")
+
+        def __subclasscheck__(self, cls):
+            raise TypeError(f"{self} cannot be used with issubclass().")
+
+    Self = _Self(_root=True)