gh-133960: Improve typing.evaluate_forward_ref (#133961)

As explained in #133960, this removes most of the behavior differences with ForwardRef.evaluate.
The remaining difference is about recursive evaluation of forwardrefs; this is practically useful
in cases where an annotation refers to a type alias that itself is string-valued.

This also improves several edge cases that were previously not handled optimally. For example,
the function now takes advantage of the partial evaluation behavior of ForwardRef.evaluate() to
evaluate more ForwardRefs in the FORWARDREF format.

This also fixes #133959 as a side effect, because the buggy behavior in #133959 derives from
evaluate_forward_ref().

Author: JelleZijlstra

Doc/library/typing.rst

@@ -3500,20 +3500,11 @@ Introspection helpers
    Evaluate an :class:`annotationlib.ForwardRef` as a :term:`type hint`.
 
    This is similar to calling :meth:`annotationlib.ForwardRef.evaluate`,
-   but unlike that method, :func:`!evaluate_forward_ref` also:
-
-   * Recursively evaluates forward references nested within the type hint.
-   * Raises :exc:`TypeError` when it encounters certain objects that are
-     not valid type hints.
-   * Replaces type hints that evaluate to :const:`!None` with
-     :class:`types.NoneType`.
-   * Supports the :attr:`~annotationlib.Format.FORWARDREF` and
-     :attr:`~annotationlib.Format.STRING` formats.
+   but unlike that method, :func:`!evaluate_forward_ref` also
+   recursively evaluates forward references nested within the type hint.
 
    See the documentation for :meth:`annotationlib.ForwardRef.evaluate` for
-   the meaning of the *owner*, *globals*, *locals*, and *type_params* parameters.
-   *format* specifies the format of the annotation and is a member of
-   the :class:`annotationlib.Format` enum.
+   the meaning of the *owner*, *globals*, *locals*, *type_params*, and *format* parameters.
 
    .. versionadded:: 3.14
 

Lib/test/test_typing.py

@@ -6859,12 +6859,10 @@ def test_forward_ref_and_final(self):
         self.assertEqual(hints, {'value': Final})
 
     def test_top_level_class_var(self):
-        # https://bugs.python.org/issue45166
-        with self.assertRaisesRegex(
-            TypeError,
-            r'typing.ClassVar\[int\] is not valid as type argument',
-        ):
-            get_type_hints(ann_module6)
+        # This is not meaningful but we don't raise for it.
+        # https://github.com/python/cpython/issues/133959
+        hints = get_type_hints(ann_module6)
+        self.assertEqual(hints, {'wrong': ClassVar[int]})
 
     def test_get_type_hints_typeddict(self):
         self.assertEqual(get_type_hints(TotalMovie), {'title': str, 'year': int})
@@ -6967,6 +6965,11 @@ def foo(a: 'Callable[..., T]'):
         self.assertEqual(get_type_hints(foo, globals(), locals()),
                          {'a': Callable[..., T]})
 
+    def test_special_forms_no_forward(self):
+        def f(x: ClassVar[int]):
+            pass
+        self.assertEqual(get_type_hints(f), {'x': ClassVar[int]})
+
     def test_special_forms_forward(self):
 
         class C:
@@ -6982,8 +6985,9 @@ class CF:
         self.assertEqual(get_type_hints(C, globals())['b'], Final[int])
         self.assertEqual(get_type_hints(C, globals())['x'], ClassVar)
         self.assertEqual(get_type_hints(C, globals())['y'], Final)
-        with self.assertRaises(TypeError):
-            get_type_hints(CF, globals()),
+        lfi = get_type_hints(CF, globals())['b']
+        self.assertIs(get_origin(lfi), list)
+        self.assertEqual(get_args(lfi), (Final[int],))
 
     def test_union_forward_recursion(self):
         ValueList = List['Value']
@@ -7216,33 +7220,113 @@ class C(Generic[T]): pass
 class EvaluateForwardRefTests(BaseTestCase):
     def test_evaluate_forward_ref(self):
         int_ref = ForwardRef('int')
-        missing = ForwardRef('missing')
+        self.assertIs(typing.evaluate_forward_ref(int_ref), int)
         self.assertIs(
             typing.evaluate_forward_ref(int_ref, type_params=()),
             int,
         )
+        self.assertIs(
+            typing.evaluate_forward_ref(int_ref, format=annotationlib.Format.VALUE),
+            int,
+        )
         self.assertIs(
             typing.evaluate_forward_ref(
-                int_ref, type_params=(), format=annotationlib.Format.FORWARDREF,
+                int_ref, format=annotationlib.Format.FORWARDREF,
             ),
             int,
         )
+        self.assertEqual(
+            typing.evaluate_forward_ref(
+                int_ref, format=annotationlib.Format.STRING,
+            ),
+            'int',
+        )
+
+    def test_evaluate_forward_ref_undefined(self):
+        missing = ForwardRef('missing')
+        with self.assertRaises(NameError):
+            typing.evaluate_forward_ref(missing)
         self.assertIs(
             typing.evaluate_forward_ref(
-                missing, type_params=(), format=annotationlib.Format.FORWARDREF,
+                missing, format=annotationlib.Format.FORWARDREF,
             ),
             missing,
         )
         self.assertEqual(
             typing.evaluate_forward_ref(
-                int_ref, type_params=(), format=annotationlib.Format.STRING,
+                missing, format=annotationlib.Format.STRING,
             ),
-            'int',
+            "missing",
         )
 
-    def test_evaluate_forward_ref_no_type_params(self):
-        ref = ForwardRef('int')
-        self.assertIs(typing.evaluate_forward_ref(ref), int)
+    def test_evaluate_forward_ref_nested(self):
+        ref = ForwardRef("int | list['str']")
+        self.assertEqual(
+            typing.evaluate_forward_ref(ref),
+            int | list[str],
+        )
+        self.assertEqual(
+            typing.evaluate_forward_ref(ref, format=annotationlib.Format.FORWARDREF),
+            int | list[str],
+        )
+        self.assertEqual(
+            typing.evaluate_forward_ref(ref, format=annotationlib.Format.STRING),
+            "int | list['str']",
+        )
+
+        why = ForwardRef('"\'str\'"')
+        self.assertIs(typing.evaluate_forward_ref(why), str)
+
+    def test_evaluate_forward_ref_none(self):
+        none_ref = ForwardRef('None')
+        self.assertIs(typing.evaluate_forward_ref(none_ref), None)
+
+    def test_globals(self):
+        A = "str"
+        ref = ForwardRef('list[A]')
+        with self.assertRaises(NameError):
+            typing.evaluate_forward_ref(ref)
+        self.assertEqual(
+            typing.evaluate_forward_ref(ref, globals={'A': A}),
+            list[str],
+        )
+
+    def test_owner(self):
+        ref = ForwardRef("A")
+
+        with self.assertRaises(NameError):
+            typing.evaluate_forward_ref(ref)
+
+        # We default to the globals of `owner`,
+        # so it no longer raises `NameError`
+        self.assertIs(
+            typing.evaluate_forward_ref(ref, owner=Loop), A
+        )
+
+    def test_inherited_owner(self):
+        # owner passed to evaluate_forward_ref
+        ref = ForwardRef("list['A']")
+        self.assertEqual(
+            typing.evaluate_forward_ref(ref, owner=Loop),
+            list[A],
+        )
+
+        # owner set on the ForwardRef
+        ref = ForwardRef("list['A']", owner=Loop)
+        self.assertEqual(
+            typing.evaluate_forward_ref(ref),
+            list[A],
+        )
+
+    def test_partial_evaluation(self):
+        ref = ForwardRef("list[A]")
+        with self.assertRaises(NameError):
+            typing.evaluate_forward_ref(ref)
+
+        self.assertEqual(
+            typing.evaluate_forward_ref(ref, format=annotationlib.Format.FORWARDREF),
+            list[EqualToForwardRef('A')],
+        )
 
 
 class CollectionsAbcTests(BaseTestCase):

Lib/typing.py

@@ -956,12 +956,8 @@ def evaluate_forward_ref(
     """Evaluate a forward reference as a type hint.
 
     This is similar to calling the ForwardRef.evaluate() method,
-    but unlike that method, evaluate_forward_ref() also:
-
-    * Recursively evaluates forward references nested within the type hint.
-    * Rejects certain objects that are not valid type hints.
-    * Replaces type hints that evaluate to None with types.NoneType.
-    * Supports the *FORWARDREF* and *STRING* formats.
+    but unlike that method, evaluate_forward_ref() also
+    recursively evaluates forward references nested within the type hint.
 
     *forward_ref* must be an instance of ForwardRef. *owner*, if given,
     should be the object that holds the annotations that the forward reference
@@ -981,23 +977,24 @@ def evaluate_forward_ref(
     if forward_ref.__forward_arg__ in _recursive_guard:
         return forward_ref
 
-    try:
-        value = forward_ref.evaluate(globals=globals, locals=locals,
-                                     type_params=type_params, owner=owner)
-    except NameError:
-        if format == _lazy_annotationlib.Format.FORWARDREF:
-            return forward_ref
-        else:
-            raise
-
-    type_ = _type_check(
-        value,
-        "Forward references must evaluate to types.",
-        is_argument=forward_ref.__forward_is_argument__,
-        allow_special_forms=forward_ref.__forward_is_class__,
-    )
+    if format is None:
+        format = _lazy_annotationlib.Format.VALUE
+    value = forward_ref.evaluate(globals=globals, locals=locals,
+                                 type_params=type_params, owner=owner, format=format)
+
+    if (isinstance(value, _lazy_annotationlib.ForwardRef)
+            and format == _lazy_annotationlib.Format.FORWARDREF):
+        return value
+
+    if isinstance(value, str):
+        value = _make_forward_ref(value, module=forward_ref.__forward_module__,
+                                  owner=owner or forward_ref.__owner__,
+                                  is_argument=forward_ref.__forward_is_argument__,
+                                  is_class=forward_ref.__forward_is_class__)
+    if owner is None:
+        owner = forward_ref.__owner__
     return _eval_type(
-        type_,
+        value,
         globals,
         locals,
         type_params,
@@ -2338,12 +2335,12 @@ def get_type_hints(obj, globalns=None, localns=None, include_extras=False,
                 # This only affects ForwardRefs.
                 base_globals, base_locals = base_locals, base_globals
             for name, value in ann.items():
-                if value is None:
-                    value = type(None)
                 if isinstance(value, str):
                     value = _make_forward_ref(value, is_argument=False, is_class=True)
                 value = _eval_type(value, base_globals, base_locals, base.__type_params__,
                                    format=format, owner=obj)
+                if value is None:
+                    value = type(None)
                 hints[name] = value
         if include_extras or format == Format.STRING:
             return hints
@@ -2377,8 +2374,6 @@ def get_type_hints(obj, globalns=None, localns=None, include_extras=False,
         localns = globalns
     type_params = getattr(obj, "__type_params__", ())
     for name, value in hints.items():
-        if value is None:
-            value = type(None)
         if isinstance(value, str):
             # class-level forward refs were handled above, this must be either
             # a module-level annotation or a function argument annotation
@@ -2387,7 +2382,10 @@ def get_type_hints(obj, globalns=None, localns=None, include_extras=False,
                 is_argument=not isinstance(obj, types.ModuleType),
                 is_class=False,
             )
-        hints[name] = _eval_type(value, globalns, localns, type_params, format=format, owner=obj)
+        value = _eval_type(value, globalns, localns, type_params, format=format, owner=obj)
+        if value is None:
+            value = type(None)
+        hints[name] = value
     return hints if include_extras else {k: _strip_annotations(t) for k, t in hints.items()}
 
 

Misc/NEWS.d/next/Library/2025-05-12-20-38-57.gh-issue-133960.Aee79f.rst

@@ -0,0 +1,3 @@
+Simplify and improve :func:`typing.evaluate_forward_ref`. It now no longer
+raises errors on certain invalid types. In several situations, it is now
+able to evaluate forward references that were previously unsupported.