Make itertools.combinations_with_replacement thread-safe

python · colesbury · Feb 4, 2026 · Feb 2, 2026 · Feb 2, 2026 · Feb 2, 2026
commit 56b615eacedbba719e688877e9ea3df0e0a86137
diff --git a/Lib/test/test_free_threading/test_itertools.py b/Lib/test/test_free_threading/test_itertools.py
@@ -1,6 +1,6 @@
 import unittest
 from threading import Thread, Barrier
-from itertools import batched, chain, cycle
+from itertools import batched, chain, combinations_with_replacement, cycle
 from test.support import threading_helper
 
 
@@ -89,6 +89,32 @@ def work(it):
 
             barrier.reset()
 
+    @threading_helper.reap_threads
+    def test_combinations_with_replacement(self):
+        number_of_threads = 6
+        number_of_iterations = 100
+        data = tuple(range(3))
+
+        barrier = Barrier(number_of_threads)
+        def work(it):
+            barrier.wait()
+            while True:
+                try:
+                    v = next(it)
+                except StopIteration:
+                    break
+
+        for _ in range(number_of_iterations):
+            cwr_iterator = combinations_with_replacement(data, 2)
+            worker_threads = []
+            for _ in range(number_of_threads):
+                worker_threads.append(
+                    Thread(target=work, args=[cwr_iterator]))
+
+            with threading_helper.start_threads(worker_threads):
+                pass
+
+            barrier.reset()
 
 
 if __name__ == "__main__":

@@ -2587,7 +2587,7 @@ cwr_traverse(PyObject *op, visitproc visit, void *arg)
 }
 
 static PyObject *
-cwr_next(PyObject *op)
+cwr_next_lock_held(PyObject *op)
 {
     cwrobject *co = cwrobject_CAST(op);
     PyObject *elem;
@@ -2666,6 +2666,16 @@ cwr_next(PyObject *op)
     return NULL;
 }
 
+static PyObject *
+cwr_next(PyObject *op)
+{
+    PyObject *result;
+    Py_BEGIN_CRITICAL_SECTION(op);
+    result = cwr_next_lock_held(op);
+    Py_END_CRITICAL_SECTION()
+    return result;
+}
+
 static PyMethodDef cwr_methods[] = {
     {"__sizeof__", cwr_sizeof, METH_NOARGS, sizeof_doc},
     {NULL,              NULL}   /* sentinel */