diff -r bffb1b7a3293 Objects/dictobject.c
--- a/Objects/dictobject.c	Thu Dec 15 19:44:28 2016 +0900
+++ b/Objects/dictobject.c	Thu Dec 15 20:03:30 2016 +0900
@@ -4296,10 +4296,23 @@ int
             }
         }
         else {
-            int shared = cached == ((PyDictObject *)dict)->ma_keys;
+            int was_shared = cached == ((PyDictObject *)dict)->ma_keys;
             res = PyDict_SetItem(dict, key, value);
-            if (shared && cached != ((PyDictObject *)dict)->ma_keys) {
-                /* Either update tp->ht_cached_keys or delete it */
+            if (was_shared && cached != ((PyDictObject *)dict)->ma_keys) {
+                /* PyDict_SetItem() may call dictresize and convert split table
+                 * into combined table.  In such case, convert it to split
+                 * table again and update type's shared key only when this is
+                 * the only dict sharing key with the type.
+                 *
+                 * This is to allow using shared key in class like this:
+                 *
+                 *     class C:
+                 *         def __init__(self):
+                 *             # one dict resize happens
+                 *             self.a, self.b, self.c = 1, 2, 3
+                 *             self.d, self.e, self.f = 4, 5, 6
+                 *     a = C()
+                 */
                 if (cached->dk_refcnt == 1) {
                     CACHED_KEYS(tp) = make_keys_shared(dict);
                 }