diff -r e3217efa6edd Lib/test/test_itertools.py
--- a/Lib/test/test_itertools.py        Sat Apr 19 13:24:03 2014 -0700
+++ b/Lib/test/test_itertools.py        Mon Apr 28 23:07:19 2014 +0400
@@ -792,6 +792,15 @@
         self.assertEqual(list(islice(c, 1, 3, 50)), [1])
         self.assertEqual(next(c), 3)

+        # Issue #21321: check source iterator is not referenced
+        # from islice() after the latter has been exhausted
+        a = [random.random() for i in range(10)]
+        before = sys.getrefcount(a)
+        b = islice(a, 5)
+        for i in b: pass
+        after = sys.getrefcount(a)
+        self.assertEqual(before, after)
+
     def test_takewhile(self):
         data = [1, 3, 5, 20, 2, 4, 6, 8]
         underten = lambda x: x<10
diff -r e3217efa6edd Modules/itertoolsmodule.c
--- a/Modules/itertoolsmodule.c Sat Apr 19 13:24:03 2014 -0700
+++ b/Modules/itertoolsmodule.c Mon Apr 28 23:07:19 2014 +0400
@@ -1241,19 +1241,22 @@
     Py_ssize_t oldnext;
     PyObject *(*iternext)(PyObject *);

+    if (it == NULL)
+        return NULL;
+
     iternext = *Py_TYPE(it)->tp_iternext;
     while (lz->cnt < lz->next) {
         item = iternext(it);
         if (item == NULL)
-            return NULL;
+            goto empty;
         Py_DECREF(item);
         lz->cnt++;
     }
     if (stop != -1 && lz->cnt >= stop)
-        return NULL;
+        goto empty;
     item = iternext(it);
     if (item == NULL)
-        return NULL;
+        goto empty;
     lz->cnt++;
     oldnext = lz->next;
     /* The (size_t) cast below avoids the danger of undefined
@@ -1262,6 +1265,10 @@
     if (lz->next < oldnext || (stop != -1 && lz->next > stop))
         lz->next = stop;
     return item;
+
+empty:
+    Py_CLEAR(lz->it);
+    return NULL;
 }

 PyDoc_STRVAR(islice_doc,