python · colesbury · Nov 19, 2025 · Nov 15, 2025 · Nov 15, 2025 · Nov 15, 2025
diff --git a/Misc/NEWS.d/next/Core_and_Builtins/2025-11-15-23-58-23.gh-issue-139103.9cVYJ0.rst b/Misc/NEWS.d/next/Core_and_Builtins/2025-11-15-23-58-23.gh-issue-139103.9cVYJ0.rst
@@ -0,0 +1 @@
+Improve multithreaded scaling of dataclasses on the free-threaded build.
@@ -6546,6 +6546,18 @@ type_setattro(PyObject *self, PyObject *name, PyObject *value)
     assert(!_PyType_HasFeature(metatype, Py_TPFLAGS_INLINE_VALUES));
     assert(!_PyType_HasFeature(metatype, Py_TPFLAGS_MANAGED_DICT));
 
+#ifdef Py_GIL_DISABLED
+    // gh-139103: Enable deferred refcounting for functions assigned
+    // to type objects.  This is important for `dataclass.__init__`,
+    // which is generated dynamically.
+    if (value != NULL &&
+        PyFunction_Check(value) &&
+        !_PyObject_HasDeferredRefcount(value))
+    {
+        PyUnstable_Object_EnableDeferredRefcount(value);
+    }
+#endif
+
     PyObject *old_value = NULL;
     PyObject *descr = _PyType_LookupRef(metatype, name);
     if (descr != NULL) {

diff --git a/Tools/ftscalingbench/ftscalingbench.py b/Tools/ftscalingbench/ftscalingbench.py
@@ -27,6 +27,7 @@
 import sys
 import threading
 import time
+from dataclasses import dataclass
 from operator import methodcaller
 
 # The iterations in individual benchmarks are scaled by this factor.
@@ -202,6 +203,17 @@ def method_caller():
     for i in range(1000 * WORK_SCALE):
         mc(obj)
 
+@dataclass
+class MyDataClass:
+    x: int
+    y: int
+    z: int
+
+@register_benchmark
+def instantiate_dataclass():
+    for _ in range(1000 * WORK_SCALE):
+        obj = MyDataClass(x=1, y=2, z=3)
+
 def bench_one_thread(func):
     t0 = time.perf_counter_ns()
     func()
Original file line number	Diff line number	Diff line change
		@@ -0,0 +1 @@
		Improve multithreaded scaling of dataclasses on the free-threaded build.