diff -r 089146b8ccc6 Lib/test/test_deque.py
--- a/Lib/test/test_deque.py	Fri Sep 25 00:49:44 2015 -0400
+++ b/Lib/test/test_deque.py	Fri Sep 25 12:04:32 2015 +0300
@@ -474,13 +474,14 @@ class TestBasic(unittest.TestCase):
         self.assertRaises(IndexError, d.popleft)
 
     def test_clear(self):
-        d = deque(range(100))
-        self.assertEqual(len(d), 100)
-        d.clear()
-        self.assertEqual(len(d), 0)
-        self.assertEqual(list(d), [])
-        d.clear()               # clear an emtpy deque
-        self.assertEqual(list(d), [])
+        for n in range(1, 101):
+            d = deque(range(n))
+            self.assertEqual(len(d), n)
+            d.clear()
+            self.assertEqual(len(d), 0)
+            self.assertEqual(list(d), [])
+            d.clear()               # clear an emtpy deque
+            self.assertEqual(list(d), [])
 
     def test_remove(self):
         d = deque('abcdefghcij')
diff -r 089146b8ccc6 Modules/_collectionsmodule.c
--- a/Modules/_collectionsmodule.c	Fri Sep 25 00:49:44 2015 -0400
+++ b/Modules/_collectionsmodule.c	Fri Sep 25 12:04:32 2015 +0300
@@ -1045,16 +1045,55 @@ PyDoc_STRVAR(remove_doc,
 static void
 deque_clear(dequeobject *deque)
 {
+    block *b;
+    block *prevblock;
+    block *leftblock = deque->leftblock;
+    Py_ssize_t leftindex = deque->leftindex;
+    Py_ssize_t n = Py_SIZE(deque);
     PyObject *item;
+    block tmp; /* a copy of right block */
 
-    while (Py_SIZE(deque)) {
-        item = deque_pop(deque, NULL);
-        assert (item != NULL);
+    /* During the process of clearing a deque, decrefs can cause the
+       deque to mutate.  To avoid fatal confusion, we have to make the
+       deque empty before clearing the blocks and never refer to
+       anything via deque->ref while clearing.  (This is the same
+       technique used in the clearing functions for lists, sets, and
+       dicts.)
+    */
+
+    b = deque->rightblock;
+    MARK_END(tmp.rightlink);
+    if (b == leftblock) {
+        memcpy(tmp.data + leftindex, b->data + leftindex,
+               sizeof(PyObject *) * (deque->rightindex + 1 - leftindex));
+        leftblock = &tmp;
+    }
+    else {
+        memcpy(tmp.data, b->data,
+               sizeof(PyObject *) * (deque->rightindex + 1));
+        b->leftlink->rightlink = &tmp;
+    }
+    MARK_END(b->leftlink);
+    MARK_END(b->rightlink);
+    Py_SIZE(deque) = 0;
+    deque->leftblock = b;
+    deque->rightblock = b;
+    deque->leftindex = CENTER + 1;
+    deque->rightindex = CENTER;
+    deque->state++;
+
+    /* Now use the old size, leftblock, and leftindex to decref the pointers */
+    while (n--) {
+        item = leftblock->data[leftindex];
         Py_DECREF(item);
+        leftindex++;
+        if (leftindex == BLOCKLEN && n) {
+            prevblock = leftblock;
+            leftblock = leftblock->rightlink;
+            leftindex = 0;
+            freeblock(prevblock);
+        }
     }
-    assert(deque->leftblock == deque->rightblock);
-    assert(deque->leftindex - 1 == deque->rightindex);
-    assert(Py_SIZE(deque) == 0);
 }
 
 static int