Support and test generics as annotate callables

Author: dr-carlos

Lib/annotationlib.py

@@ -900,7 +900,9 @@ def _get_annotate_attr(annotate, attr, default):
         if call_func := getattr(annotate.__call__, "__func__", None):
             return getattr(call_func, attr, default)
 
-    if isinstance(annotate, type):
+    # Classes and generics are callable, usually the __init__ method sets attributes,
+    # so let's access this method for fake globals and the like.
+    if isinstance(annotate, type) or isinstance(annotate, types.GenericAlias):
         return getattr(annotate.__init__, attr, default)
 
     if (wrapped := getattr(annotate, "__wrapped__", None)) is not None:
@@ -936,6 +938,17 @@ def _direct_call_annotate(func, annotate, format):
         func(inst, format)
         return inst
 
+    # Generic instantiation is slightly different.
+    if isinstance(annotate, types.GenericAlias):
+        inst = annotate.__new__(annotate.__origin__)
+        func(inst, format)
+        # Try to set the original class on the instance, if possible.
+        try:
+            inst.__orig_class__ = annotate
+        except Exception:
+            pass
+        return inst
+
     if functools := sys.modules.get("functools", None):
         # If annotate is a partial function, re-create it with the new function object.
         # We could call the function directly, but then we'd have to handle placeholders,

Lib/test/test_annotationlib.py

@@ -1687,6 +1687,25 @@ def __init__(self, format, /, __Format=Format,
 
         self.assertEqual(annotations, {"x": int})
 
+    def test_callable_generic_class_annotate_forwardref_value_fallback(self):
+        # If Format.STRING and Format.VALUE_WITH_FAKE_GLOBALS are not
+        # supported fall back to Format.VALUE and convert to strings
+        class Annotate[T](dict[T]):
+            def __init__(self, format, /, __Format=Format,
+                         __NotImplementedError=NotImplementedError):
+                if format == __Format.VALUE:
+                    super().__init__({"x": int})
+                else:
+                    raise __NotImplementedError(format)
+
+        annotations = annotationlib.call_annotate_function(
+            Annotate[int],
+            Format.FORWARDREF,
+        )
+
+        self.assertEqual(annotations, {"x": int})
+        self.assertEqual(annotations.__orig_class__, Annotate[int])
+
     def test_callable_partial_annotate_forwardref_value_fallback(self):
         # If Format.STRING and Format.VALUE_WITH_FAKE_GLOBALS are not
         # supported fall back to Format.VALUE and convert to strings