gh-152166: Fix array.array.fromlist() exposing uninitialized memory when an element's __index__ resizes the array

Lib/test/test_array.py

@@ -82,6 +82,28 @@ def __index__(self):
                 with self.assertRaises(TypeError):
                     a.fromlist(lst)
 
+    def test_fromlist_reentrant_self_resize(self):
+        # gh-152166: if an element's __index__ resizes the array being
+        # filled, fromlist() must not skip the preallocated slots (which
+        # exposes uninitialized memory) or misplace items.  It raises
+        # RuntimeError and rolls back, mirroring the list-mutation guard.
+        for label, mutate in (("grow", lambda a: a.append(0)),
+                              ("shrink", lambda a: a.pop())):
+            for typecode in ('i', 'I', 'l', 'L', 'q', 'Q'):
+                with self.subTest(typecode=typecode, mutate=label):
+                    a = array.array(typecode, [1, 2, 3])
+                    before = a.tolist()
+
+                    class Evil:
+                        def __index__(self, _a=a, _m=mutate):
+                            _m(_a)
+                            return 0
+
+                    with self.assertRaises(RuntimeError):
+                        a.fromlist([Evil(), 4, 5])
+                    # The failed call must leave the array unchanged.
+                    self.assertEqual(a.tolist(), before)
+
     def test_typecodes(self):
         self.assertIsInstance(array.typecodes, tuple)
         for typecode in array.typecodes:

Misc/NEWS.d/next/Library/2026-06-25-14-03-16.gh-issue-152166.hH7w1v.rst

@@ -0,0 +1,4 @@
+Fix :meth:`array.array.fromlist` exposing uninitialized memory (and misplacing
+items) when an element's :meth:`~object.__index__` method resizes the array
+during the conversion.  The reserved slots are now filled at a fixed offset and
+the operation raises :exc:`RuntimeError` if the array is resized mid-iteration.

Modules/arraymodule.c

@@ -1805,8 +1805,12 @@ array_array_fromlist_impl(arrayobject *self, PyObject *list)
             return NULL;
         for (i = 0; i < n; i++) {
             PyObject *v = PyList_GET_ITEM(list, i);
-            if ((*self->ob_descr->setitem)(self,
-                            Py_SIZE(self) - n + i, v) != 0) {
+            /* setitem may run arbitrary Python (e.g. __index__), which can
+               resize self.  Write to the fixed slot reserved above rather
+               than an offset recomputed from the live Py_SIZE, and bail out
+               if self was resized -- otherwise we would skip a preallocated
+               slot (exposing uninitialized memory) or misplace items. */
+            if ((*self->ob_descr->setitem)(self, old_size + i, v) != 0) {
                 array_resize(self, old_size);
                 return NULL;
             }
@@ -1816,6 +1820,12 @@ array_array_fromlist_impl(arrayobject *self, PyObject *list)
                 array_resize(self, old_size);
                 return NULL;
             }
+            if (Py_SIZE(self) != old_size + n) {
+                PyErr_SetString(PyExc_RuntimeError,
+                                "array changed size during iteration");
+                array_resize(self, old_size);
+                return NULL;
+            }
         }
     }
     Py_RETURN_NONE;