Assure UnionType stays UnionType; check repr only

+ updated comments

Author: Daraan

src/test_typing_extensions.py

@@ -1679,7 +1679,7 @@ def test_annotation_and_optional_default(self):
             Union["annotation", T_default]     : Union[annotation, T_default],
             Annotated["annotation", "nested"]  : Annotated[Union[int, None], "data", "nested"],
         }
-        if TYPING_3_10_0:  # cannot construct UnionTypes
+        if TYPING_3_10_0:  # cannot construct UnionTypes before 3.10
             do_not_stringify_cases["str | NoneAlias | StrAlias"] = str | None
             cases[str | None] = Optional[str]
         cases.update(do_not_stringify_cases)
@@ -1690,7 +1690,7 @@ def test_annotation_and_optional_default(self):
             skip_reason = None
             annot_unchanged = annot
             if sys.version_info[:2] == (3, 10) and annot == "str | NoneAlias | StrAlias" and none_default:
-                # different repr here as Optional[str | None] -> Optional[str] not a UnionType
+                # In 3.10 converts Optional[str | None] to Optional[str] which has a different repr
                 skip_reason = "UnionType not preserved in 3.10"
             if wrap_optional:
                 if annot_unchanged == ():
@@ -1726,11 +1726,13 @@ def func(x: annot): pass
                 # Hash
                 for k in type_hints.keys():
                     self.assertEqual(hash(type_hints[k]), hash(expected[k]))
+                    # Test if UnionTypes are preserved
+                    self.assertEqual(isinstance(type_hints[k], type(expected[k])), True)
                 # Repr
                 with self.subTest("Check str and repr"):
                     if skip_reason == "UnionType not preserved in 3.10":
                         self.skipTest(skip_reason)
-                    self.assertEqual(str(type_hints) + repr(type_hints), str(expected) + repr(expected))
+                    self.assertEqual(repr(type_hints), repr(expected))
 
 
 class GetUtilitiesTestCase(TestCase):

src/typing_extensions.py

@@ -1282,7 +1282,7 @@ def _clean_optional(obj, hints, globalns=None, localns=None):
             ):
                 continue
             original_value = original_hints[name]
-            if original_value is None:  # should not happen
+            if original_value is None:  # should be NoneType already; check just in case
                 original_value = _NoneType
             # Forward reference
             if isinstance(original_value, str):
@@ -1310,8 +1310,14 @@ def _clean_optional(obj, hints, globalns=None, localns=None):
             if sys.version_info < (3, 9) and get_origin(original_evaluated) is Union:
                 # Union[str, None, "str"] is not reduced to Union[str, None]
                 original_evaluated = Union[original_evaluated.__args__]
-            # Compare if values differ
-            if original_evaluated != value:
+            # Compare if values differ. Note that even if equal
+            # value might be cached by typing._tp_cache contrary to original_evaluated
+            if original_evaluated != value or (
+                # 3.10: ForwardRefs of UnionType might be turned into _UnionGenericAlias
+                hasattr(_types, "UnionType")
+                and isinstance(original_evaluated, _types.UnionType)
+                and not isinstance(value, _types.UnionType)
+            ):
                 hints[name] = original_evaluated
 
 # Python 3.9+ has PEP 593 (Annotated)