gh-159739: Preserve the full maxlen when copying deque subclasses (#159765)

Co-authored-by: Jelle Zijlstra <jelle.zijlstra@gmail.com>

Author: jelle-openai

Lib/test/test_deque.py

@@ -810,6 +810,12 @@ def __iter__(self):
 
 class TestSubclass(unittest.TestCase):
 
+    def test_copy_maxlen(self):
+        original = Deque('abc', maxlen=support.MAX_Py_ssize_t)
+        copied = original.copy()
+        self.assertEqual(copied, original)
+        self.assertEqual(copied.maxlen, original.maxlen)
+
     def test_basics(self):
         d = Deque(range(25))
         d.__init__(range(200))

Misc/NEWS.d/next/Library/2026-10-19-20-38-17.gh-issue-159739.ff088c.rst

@@ -0,0 +1 @@
+Fix copying :class:`collections.deque` subclasses with a large maximum length.

Modules/_collectionsmodule.c

@@ -648,7 +648,7 @@ deque_copy_impl(dequeobject *deque)
         result = PyObject_CallOneArg((PyObject *)(Py_TYPE(deque)),
                                      (PyObject *)deque);
     else
-        result = PyObject_CallFunction((PyObject *)(Py_TYPE(deque)), "Oi",
+        result = PyObject_CallFunction((PyObject *)(Py_TYPE(deque)), "On",
                                        deque, old_deque->maxlen, NULL);
     if (result != NULL && !PyObject_TypeCheck(result, state->deque_type)) {
         PyErr_Format(PyExc_TypeError,