bpo-46539: Pass status of special typeforms to forward references (GH-30926)

miss-islington · GBeauregard · web-flow · commit bfcb41420a32 · 2022-01-27T08:48:08.000-08:00
Previously this didn't matter because there weren't any valid code paths that could trigger a type check with a special form, but after the bug fix for `Annotated` wrapping special forms it's now possible to annotate something like `Annotated['ClassVar[int]', (3, 4)]`. This change would also be needed for proposed future changes, such as allowing `ClassVar` and `Final` to nest each other in dataclasses. (cherry picked from commit ced5005) Co-authored-by: Gregory Beauregard <greg@greg.red>
diff --git a/Lib/test/test_typing.py b/Lib/test/test_typing.py
@@ -2809,6 +2809,20 @@ def foo(a: 'Callable[..., T]'):
         self.assertEqual(get_type_hints(foo, globals(), locals()),
                          {'a': Callable[..., T]})
 
+    def test_special_forms_forward(self):
+
+        class C:
+            a: Annotated['ClassVar[int]', (3, 5)] = 4
+            b: Annotated['Final[int]', "const"] = 4
+
+        class CF:
+            b: List['Final[int]'] = 4
+
+        self.assertEqual(get_type_hints(C, globals())['a'], ClassVar[int])
+        self.assertEqual(get_type_hints(C, globals())['b'], Final[int])
+        with self.assertRaises(TypeError):
+            get_type_hints(CF, globals()),
+
     def test_syntax_error(self):
 
         with self.assertRaises(SyntaxError):
diff --git a/Lib/typing.py b/Lib/typing.py
@@ -134,12 +134,12 @@
 # legitimate imports of those modules.
 
 
-def _type_convert(arg, module=None):
+def _type_convert(arg, module=None, *, allow_special_forms=False):
     """For converting None to type(None), and strings to ForwardRef."""
     if arg is None:
         return type(None)
     if isinstance(arg, str):
-        return ForwardRef(arg, module=module)
+        return ForwardRef(arg, module=module, is_class=allow_special_forms)
     return arg
 
 
@@ -161,7 +161,7 @@ def _type_check(arg, msg, is_argument=True, module=None, *, allow_special_forms=
         if is_argument:
             invalid_generic_forms += (Final,)
 
-    arg = _type_convert(arg, module=module)
+    arg = _type_convert(arg, module=module, allow_special_forms=allow_special_forms)
     if (isinstance(arg, _GenericAlias) and
             arg.__origin__ in invalid_generic_forms):
         raise TypeError(f"{arg} is not valid as type argument")
diff --git a/Misc/NEWS.d/next/Library/2022-01-26-20-36-30.bpo-46539.23iW1d.rst b/Misc/NEWS.d/next/Library/2022-01-26-20-36-30.bpo-46539.23iW1d.rst
@@ -0,0 +1 @@
+In :func:`typing.get_type_hints`, support evaluating stringified ``ClassVar`` and ``Final`` annotations inside ``Annotated``. Patch by Gregory Beauregard.

Original file line number	Diff line number	Diff line change
`@@ -0,0 +1 @@`
	`1`	+In :func:`typing.get_type_hints`, support evaluating stringified ``ClassVar`` and ``Final`` annotations inside ``Annotated``. Patch by Gregory Beauregard.