Add PEP 613 TypeAlias to typing_extensions

typing_extensions/src_py2/test_typing_extensions.py

@@ -7,7 +7,7 @@
 
 from typing_extensions import Annotated, NoReturn, ClassVar, IntVar
 from typing_extensions import ContextManager, Counter, Deque, DefaultDict
-from typing_extensions import NewType, overload
+from typing_extensions import NewType, TypeAlias, overload
 from typing import Dict, List
 import typing
 import typing_extensions
@@ -377,6 +377,47 @@ def test_annotated_in_other_types(self):
         self.assertEqual(X[int], List[Annotated[int, 5]])
 
 
+class TypeAliasTests(BaseTestCase):
+    def test_canonical_usage(self):
+        Alias = Employee  # type: TypeAlias
+
+    def test_cannot_instantiate(self):
+        with self.assertRaises(TypeError):
+            TypeAlias()
+
+    def test_no_isinstance(self):
+        with self.assertRaises(TypeError):
+            isinstance(42, TypeAlias)
+
+    def test_no_issubclass(self):
+        with self.assertRaises(TypeError):
+            issubclass(Employee, TypeAlias)
+
+        with self.assertRaises(TypeError):
+            issubclass(TypeAlias, Employee)
+
+    def test_cannot_subclass(self):
+        with self.assertRaises(TypeError):
+            class C(TypeAlias):
+                pass
+
+        with self.assertRaises(TypeError):
+            class C(type(TypeAlias)):
+                pass
+
+    def test_repr(self):
+        if hasattr(typing, 'TypeAlias'):
+            self.assertEqual(repr(TypeAlias), 'typing.TypeAlias')
+            self.assertEqual(repr(type(TypeAlias)), 'typing.TypeAlias')
+        else:
+            self.assertEqual(repr(TypeAlias), 'typing_extensions.TypeAlias')
+            self.assertEqual(repr(type(TypeAlias)), 'typing_extensions.TypeAlias')
+
+    def test_cannot_subscript(self):
+        with self.assertRaises(TypeError):
+            TypeAlias[int]
+
+
 class AllTests(BaseTestCase):
 
     def test_typing_extensions_includes_standard(self):

typing_extensions/src_py2/typing_extensions.py

@@ -241,5 +241,43 @@ class Annotated(object):
     __slots__ = ()
 
 
+class _TypeAliasMeta(typing.TypingMeta):
+    """Metaclass for TypeAlias"""
+
+    def __new__(cls, name, bases, namespace):
+        cls.assert_no_subclassing(bases)
+        self = super(_TypeAliasMeta, cls).__new__(cls, name, bases, namespace)
+        return self
+
+    def __repr__(self):
+        return 'typing_extensions.TypeAlias'
+
+
+class _TypeAliasBase(typing._FinalTypingBase):
+    """Special marker indicating that an assignment should
+    be recognized as a proper type alias definition by type
+    checkers.
+
+    For example::
+
+        Predicate = Callable[..., bool]  # type: TypeAlias
+
+    It's invalid when used anywhere except as in the example above.
+    """
+    __metaclass__ = _TypeAliasMeta
+    __slots__ = ()
+
+    def __instancecheck__(self, obj):
+        raise TypeError("TypeAlias cannot be used with isinstance().")
+
+    def __subclasscheck__(self, cls):
+        raise TypeError("TypeAlias cannot be used with issubclass().")
+
+    def __repr__(self):
+        return 'typing_extensions.TypeAlias'
+
+
+TypeAlias = _TypeAliasBase(_root=True)
+
 # This alias exists for backwards compatibility.
 runtime = runtime_checkable

typing_extensions/src_py3/test_typing_extensions.py

@@ -13,6 +13,7 @@
 from typing import Generic
 from typing import no_type_check
 from typing_extensions import NoReturn, ClassVar, Final, IntVar, Literal, Type, NewType, TypedDict
+from typing_extensions import TypeAlias
 try:
     from typing_extensions import Protocol, runtime, runtime_checkable
 except ImportError:
@@ -1822,6 +1823,52 @@ def __iand__(self, other: Const["MySet[T]"]) -> "MySet[T]":
         )
 
 
+class TypeAliasTests(BaseTestCase):
+    @skipUnless(PY36, 'Python 3.6 required')
+    def test_canonical_usage_with_variable_annotation(self):
+        ns = {}
+        exec('Alias: TypeAlias = Employee', globals(), ns)
+
+    def test_canonical_usage_with_type_comment(self):
+        Alias = Employee  # type: TypeAlias
+
+    def test_cannot_instantiate(self):
+        with self.assertRaises(TypeError):
+            TypeAlias()
+
+    def test_no_isinstance(self):
+        with self.assertRaises(TypeError):
+            isinstance(42, TypeAlias)
+
+    def test_no_issubclass(self):
+        with self.assertRaises(TypeError):
+            issubclass(Employee, TypeAlias)
+
+        if SUBCLASS_CHECK_FORBIDDEN:
+            with self.assertRaises(TypeError):
+                issubclass(TypeAlias, Employee)
+
+    def test_cannot_subclass(self):
+        with self.assertRaises(TypeError):
+            class C(TypeAlias):
+                pass
+
+        if SUBCLASS_CHECK_FORBIDDEN:
+            with self.assertRaises(TypeError):
+                class C(type(TypeAlias)):
+                    pass
+
+    def test_repr(self):
+        if hasattr(typing, 'TypeAlias'):
+            self.assertEqual(repr(TypeAlias), 'typing.TypeAlias')
+        else:
+            self.assertEqual(repr(TypeAlias), 'typing_extensions.TypeAlias')
+
+    def test_cannot_subscript(self):
+        with self.assertRaises(TypeError):
+            TypeAlias[int]
+
+
 class AllTests(BaseTestCase):
 
     def test_typing_extensions_includes_standard(self):

typing_extensions/src_py3/typing_extensions.py

@@ -2071,3 +2071,98 @@ def get_args(tp):
                 res = (list(res[:-1]), res[-1])
             return res
         return ()
+
+
+if hasattr(typing, 'TypeAlias'):
+    TypeAlias = typing.TypeAlias
+elif sys.version_info[:2] >= (3, 9):
+    class _TypeAliasForm(typing._SpecialForm, _root=True):
+        def __repr__(self):
+            return 'typing_extensions.' + self._name
+
+    @_TypeAliasForm
+    def TypeAlias(self, parameters):
+        """Special marker indicating that an assignment should
+        be recognized as a proper type alias definition by type
+        checkers.
+
+        For example::
+
+            Predicate: TypeAlias = Callable[..., bool]
+
+        It's invalid when used anywhere except as in the example above.
+        """
+        raise TypeError("{} is not subscriptable".format(self))
+
+elif sys.version_info[:2] >= (3, 7):
+    class _TypeAliasForm(typing._SpecialForm, _root=True):
+        def __repr__(self):
+            return 'typing_extensions.' + self._name
+
+    TypeAlias = _TypeAliasForm('TypeAlias',
+                               doc="""Special marker indicating that an assignment should
+                               be recognized as a proper type alias definition by type
+                               checkers.
+
+                               For example::
+
+                                   Predicate: TypeAlias = Callable[..., bool]
+
+                               It's invalid when used anywhere except as in the example
+                               above.""")
+
+elif hasattr(typing, '_FinalTypingBase'):
+    class _TypeAliasMeta(typing.TypingMeta):
+        """Metaclass for TypeAlias"""
+
+        def __repr__(self):
+            return 'typing_extensions.TypeAlias'
+
+    class _TypeAliasBase(typing._FinalTypingBase, metaclass=_TypeAliasMeta, _root=True):
+        """Special marker indicating that an assignment should
+        be recognized as a proper type alias definition by type
+        checkers.
+
+        For example::
+
+            Predicate: TypeAlias = Callable[..., bool]
+
+        It's invalid when used anywhere except as in the example above.
+        """
+        __slots__ = ()
+
+        def __instancecheck__(self, obj):
+            raise TypeError("TypeAlias cannot be used with isinstance().")
+
+        def __subclasscheck__(self, cls):
+            raise TypeError("TypeAlias cannot be used with issubclass().")
+
+        def __repr__(self):
+            return 'typing_extensions.TypeAlias'
+
+    TypeAlias = _TypeAliasBase(_root=True)
+else:
+    class _TypeAliasMeta(typing.TypingMeta):
+        """Metaclass for TypeAlias"""
+
+        def __instancecheck__(self, obj):
+            raise TypeError("TypeAlias cannot be used with isinstance().")
+
+        def __subclasscheck__(self, cls):
+            raise TypeError("TypeAlias cannot be used with issubclass().")
+
+        def __call__(self, *args, **kwargs):
+            raise TypeError("Cannot instantiate TypeAlias")
+
+    class TypeAlias(metaclass=_TypeAliasMeta, _root=True):
+        """Special marker indicating that an assignment should
+        be recognized as a proper type alias definition by type
+        checkers.
+
+        For example::
+
+            Predicate: TypeAlias = Callable[..., bool]
+
+        It's invalid when used anywhere except as in the example above.
+        """
+        __slots__ = ()