gh-127190: Fix local_setattro() error handling

Lib/test/test_threading_local.py

@@ -208,6 +208,21 @@ def test_threading_local_clear_race(self):
 
         _testcapi.join_temporary_c_thread()
 
+    @support.cpython_only
+    def test_error(self):
+        class Loop(self._local):
+            attr = 1
+
+        # Trick the "if name == '__dict__':" test of __setattr__()
+        # to always be true
+        class NameCompareTrue:
+            def __eq__(self, other):
+                return True
+
+        loop = Loop()
+        with self.assertRaisesRegex(AttributeError, 'Loop.*read-only'):
+            loop.__setattr__(NameCompareTrue(), 2)
+
 
 class ThreadLocalTest(unittest.TestCase, BaseLocalTest):
     _local = _thread._local

Modules/_threadmodule.c

@@ -1624,7 +1624,7 @@ local_setattro(localobject *self, PyObject *name, PyObject *v)
     }
     if (r == 1) {
         PyErr_Format(PyExc_AttributeError,
-                     "'%.100s' object attribute '%U' is read-only",
+                     "'%.100s' object attribute %R is read-only",
                      Py_TYPE(self)->tp_name, name);
         goto err;
     }