bpo-41923: PEP 613: Add TypeAlias to typing module

This special marker annotation is intended to help in distinguishing proper PEP 484-compliant type aliases from regular top-level variable assignments.
python · gvanrossum · Oct 7, 2020 · Oct 3, 2020 · Oct 5, 2020 · Oct 6, 2020
commit c15b70610400ffb4233462534ee081841ce8682d
diff --git a/Doc/whatsnew/3.10.rst b/Doc/whatsnew/3.10.rst
@@ -82,6 +82,10 @@ New Features
 * :pep:`618`: The :func:`zip` function now has an optional ``strict`` flag, used
   to require that all the iterables have an equal length.
 
+* :pep:`613`: The :mod:`typing` module now has a special annotation
+  :class:`TypeAlias` to explicitly declare :pep:`484`-compliant type aliases,
+  better differentiating them from regular assignments.
+
 
 Other Language Changes
 ======================

diff --git a/Lib/test/test_typing.py b/Lib/test/test_typing.py
@@ -24,6 +24,7 @@
 from typing import IO, TextIO, BinaryIO
 from typing import Pattern, Match
 from typing import Annotated, ForwardRef
+from typing import TypeAlias
 import abc
 import typing
 import weakref
@@ -4176,6 +4177,45 @@ def test_annotated_in_other_types(self):
         self.assertEqual(X[int], List[Annotated[int, 5]])
 
 
+class TypeAliasTests(BaseTestCase):
+    def test_canonical_usage_with_variable_annotation(self):
+        Alias: TypeAlias = Employee
+
+    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)
+
+        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):
+        self.assertEqual(repr(TypeAlias), 'typing.TypeAlias')
+
+    def test_cannot_subscript(self):
+        with self.assertRaises(TypeError):
+            TypeAlias[int]
+
+
 class AllTests(BaseTestCase):
     """Tests for __all__."""
 

diff --git a/Lib/typing.py b/Lib/typing.py
@@ -112,6 +112,7 @@
     'runtime_checkable',
     'Text',
     'TYPE_CHECKING',
+    'TypeAlias',
 ]
 
 # The pseudo-submodules 're' and 'io' are part of the public
@@ -459,6 +460,21 @@ def open_helper(file: str, mode: MODE) -> str:
     return _GenericAlias(self, parameters)
 
 
+@_SpecialForm
+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(f"{self} is not subscriptable")
+
+
 class ForwardRef(_Final, _root=True):
     """Internal wrapper to hold a forward reference."""
 

diff --git a/Misc/ACKS b/Misc/ACKS
@@ -611,6 +611,7 @@ Christoph Gohlke
 Tim Golden
 Yonatan Goldschmidt
 Mark Gollahon
+Mikhail Golubev
 Guilherme Gonçalves
 Tiago Gonçalves
 Chris Gonnerman

diff --git a/Misc/NEWS.d/next/Library/2020-10-03-23-14-50.bpo-41923.Buonw9.rst b/Misc/NEWS.d/next/Library/2020-10-03-23-14-50.bpo-41923.Buonw9.rst
@@ -0,0 +1 @@
+Implement :pep:`613`, introducing :class:`typing.TypeAlias` annotation.
Original file line number	Diff line number	Diff line change
		@@ -0,0 +1 @@
		Implement :pep:`613`, introducing :class:`typing.TypeAlias` annotation.
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