Only for frozenset

Author: yoney

Misc/NEWS.d/next/Core_and_Builtins/2025-10-22-12-48-05.gh-issue-140476.F3-d1P.rst

@@ -1,2 +1,2 @@
-Optimize :c:func:`PySet_Add` for uniquely referenced sets in :term:`free
-threaded <free threading>` build.
+Optimize :c:func:`PySet_Add` for :class:`frozenset` in :term:`free threaded
+<free threading>` build.

Objects/setobject.c

@@ -2773,13 +2773,6 @@ PySet_Discard(PyObject *set, PyObject *key)
 int
 PySet_Add(PyObject *anyset, PyObject *key)
 {
-    if (_PyObject_IsUniquelyReferenced(anyset) && PyAnySet_Check(anyset)) {
-        // We can only change frozensets if they are uniquely referenced,
-        // and we can avoid locking sets even in free-threaded build if they
-        // are uniquely referenced.
-        return set_add_key((PySetObject *)anyset, key);
-    }
-
     if (PySet_Check(anyset)) {
         int rv;
         Py_BEGIN_CRITICAL_SECTION(anyset);
@@ -2788,6 +2781,14 @@ PySet_Add(PyObject *anyset, PyObject *key)
         return rv;
     }
 
+    if (PyFrozenSet_Check(anyset) || _PyObject_IsUniquelyReferenced(anyset)) {
+        // We can only change frozensets if they are uniquely referenced. The
+        // API limits the usage of `PySet_Add` to "fill in the values of brand
+        // new frozensets before they are exposed to other code". In this case,
+        // this can be done without a lock.
+        return set_add_key((PySetObject *)anyset, key);
+    }
+
     PyErr_BadInternalCall();
     return -1;
 }