Don't mutate globals!

Author: AlexWaygood

Lib/test/test_typing.py

@@ -4859,6 +4859,8 @@ def f(x: X): ...
         )
 
     def test_pep695_generic_with_future_annotations(self):
+        original_globals = dict(ann_module695.__dict__)
+
         hints_for_A = get_type_hints(ann_module695.A)
         A_type_params = ann_module695.A.__type_params__
         self.assertIs(hints_for_A["x"], A_type_params[0])
@@ -4898,6 +4900,9 @@ def test_pep695_generic_with_future_annotations(self):
             {"x": params["Foo"], "y": params["Bar"], "return": types.NoneType}
         )
 
+        # should not have changed as a result of the get_type_hints() calls!
+        self.assertEqual(ann_module695.__dict__, original_globals)
+
     def test_extended_generic_rules_subclassing(self):
         class T1(Tuple[T, KT]): ...
         class T2(Tuple[T, ...]): ...

Lib/typing.py

@@ -1068,14 +1068,16 @@ def _evaluate(self, globalns, localns, type_params=_sentinel, *, recursive_guard
             # names in the global scope (which here are called `localns`!),
             # but should in turn be overridden by names in the class scope
             # (which here are called `globalns`!)
-            type_params = {param.__name__: param for param in type_params}
-            if self.__forward_is_class__:
-                for name, param in type_params.items():
-                    if name not in globalns:
-                        globalns[name] = param
-                        localns.pop(name, None)
-            else:
-                localns = type_params | localns
+            if type_params:
+                name_to_param_map = {param.__name__: param for param in type_params}
+                if self.__forward_is_class__:
+                    globalns, localns = dict(globalns), dict(localns)
+                    for name, param in name_to_param_map.items():
+                        if name not in globalns:
+                            globalns[name] = param
+                            localns.pop(name, None)
+                else:
+                    localns = name_to_param_map | localns
 
             type_ = _type_check(
                 eval(self.__forward_code__, globalns, localns),