Add NoReturn type (#397)

* Add NoReturn type

* Simplify NoReturn checks

Author: ilevkivskyi

python2/test_typing.py

@@ -7,7 +7,7 @@
 from unittest import TestCase, main, SkipTest
 from copy import copy, deepcopy
 
-from typing import Any
+from typing import Any, NoReturn
 from typing import TypeVar, AnyStr
 from typing import T, KT, VT  # Not in __all__.
 from typing import Union, Optional
@@ -106,6 +106,40 @@ def test_any_is_subclass(self):
         typing.IO[Any]
 
 
+class NoReturnTests(BaseTestCase):
+
+    def test_noreturn_instance_type_error(self):
+        with self.assertRaises(TypeError):
+            isinstance(42, NoReturn)
+
+    def test_noreturn_subclass_type_error(self):
+        with self.assertRaises(TypeError):
+            issubclass(Employee, NoReturn)
+        with self.assertRaises(TypeError):
+            issubclass(NoReturn, Employee)
+
+    def test_repr(self):
+        self.assertEqual(repr(NoReturn), 'typing.NoReturn')
+
+    def test_not_generic(self):
+        with self.assertRaises(TypeError):
+            NoReturn[int]
+
+    def test_cannot_subclass(self):
+        with self.assertRaises(TypeError):
+            class A(NoReturn):
+                pass
+        with self.assertRaises(TypeError):
+            class A(type(NoReturn)):
+                pass
+
+    def test_cannot_instantiate(self):
+        with self.assertRaises(TypeError):
+            NoReturn()
+        with self.assertRaises(TypeError):
+            type(NoReturn)()
+
+
 class TypeVarTests(BaseTestCase):
 
     def test_basic_plain(self):

python2/typing.py

@@ -478,6 +478,40 @@ def __subclasscheck__(self, cls):
 Any = _Any(_root=True)
 
 
+class NoReturnMeta(TypingMeta):
+    """Metaclass for NoReturn."""
+
+    def __new__(cls, name, bases, namespace):
+        cls.assert_no_subclassing(bases)
+        self = super(NoReturnMeta, cls).__new__(cls, name, bases, namespace)
+        return self
+
+
+class _NoReturn(_FinalTypingBase):
+    """Special type indicating functions that never return.
+    Example::
+
+      from typing import NoReturn
+
+      def stop() -> NoReturn:
+          raise Exception('no way')
+
+    This type is invalid in other positions, e.g., ``List[NoReturn]``
+    will fail in static type checkers.
+    """
+    __metaclass__ = NoReturnMeta
+    __slots__ = ()
+
+    def __instancecheck__(self, obj):
+        raise TypeError("NoReturn cannot be used with isinstance().")
+
+    def __subclasscheck__(self, cls):
+        raise TypeError("NoReturn cannot be used with issubclass().")
+
+
+NoReturn = _NoReturn(_root=True)
+
+
 class TypeVarMeta(TypingMeta):
     def __new__(cls, name, bases, namespace):
         cls.assert_no_subclassing(bases)

src/test_typing.py

@@ -6,7 +6,7 @@
 from unittest import TestCase, main, skipUnless, SkipTest
 from copy import copy, deepcopy
 
-from typing import Any
+from typing import Any, NoReturn
 from typing import TypeVar, AnyStr
 from typing import T, KT, VT  # Not in __all__.
 from typing import Union, Optional
@@ -109,6 +109,40 @@ def test_any_works_with_alias(self):
         typing.IO[Any]
 
 
+class NoReturnTests(BaseTestCase):
+
+    def test_noreturn_instance_type_error(self):
+        with self.assertRaises(TypeError):
+            isinstance(42, NoReturn)
+
+    def test_noreturn_subclass_type_error(self):
+        with self.assertRaises(TypeError):
+            issubclass(Employee, NoReturn)
+        with self.assertRaises(TypeError):
+            issubclass(NoReturn, Employee)
+
+    def test_repr(self):
+        self.assertEqual(repr(NoReturn), 'typing.NoReturn')
+
+    def test_not_generic(self):
+        with self.assertRaises(TypeError):
+            NoReturn[int]
+
+    def test_cannot_subclass(self):
+        with self.assertRaises(TypeError):
+            class A(NoReturn):
+                pass
+        with self.assertRaises(TypeError):
+            class A(type(NoReturn)):
+                pass
+
+    def test_cannot_instantiate(self):
+        with self.assertRaises(TypeError):
+            NoReturn()
+        with self.assertRaises(TypeError):
+            type(NoReturn)()
+
+
 class TypeVarTests(BaseTestCase):
 
     def test_basic_plain(self):

src/typing.py

@@ -420,6 +420,31 @@ def __subclasscheck__(self, cls):
 Any = _Any(_root=True)
 
 
+class _NoReturn(_FinalTypingBase, _root=True):
+    """Special type indicating functions that never return.
+    Example::
+
+      from typing import NoReturn
+
+      def stop() -> NoReturn:
+          raise Exception('no way')
+
+    This type is invalid in other positions, e.g., ``List[NoReturn]``
+    will fail in static type checkers.
+    """
+
+    __slots__ = ()
+
+    def __instancecheck__(self, obj):
+        raise TypeError("NoReturn cannot be used with isinstance().")
+
+    def __subclasscheck__(self, cls):
+        raise TypeError("NoReturn cannot be used with issubclass().")
+
+
+NoReturn = _NoReturn(_root=True)
+
+
 class TypeVar(_TypingBase, _root=True):
     """Type variable.