gh-145566: Skip stop-the-world when reassigning __class__ on newly created objects

Author: colesbury

Misc/NEWS.d/next/Core_and_Builtins/2026-03-05-19-10-56.gh-issue-145566.H4RupyYN.rst

@@ -0,0 +1,2 @@
+In the free threading build, skip the stop-the-world pause when reassigning
+``__class__`` on a newly created object.

Objects/typeobject.c

@@ -7608,10 +7608,15 @@ object_set_class(PyObject *self, PyObject *value, void *closure)
         return -1;
     }
 
-    types_stop_world();
+    int unique = _PyObject_IsUniquelyReferenced(self);
+    if (!unique) {
+        types_stop_world();
+    }
     PyTypeObject *oldto = Py_TYPE(self);
     int res = object_set_class_world_stopped(self, newto);
-    types_start_world();
+    if (!unique) {
+        types_start_world();
+    }
     if (res == 0) {
         if (oldto->tp_flags & Py_TPFLAGS_HEAPTYPE) {
             Py_DECREF(oldto);