1 files changed, 16 insertions, 0 deletions
diff --git a/Modules/_collectionsmodule.c b/Modules/_collectionsmodule.c
index 9924b63..67ff55b 100644
--- a/Modules/_collectionsmodule.c
+++ b/Modules/_collectionsmodule.c
@@ -332,6 +332,14 @@ deque_extend(dequeobject *deque, PyObject *iterable)
         return result;
     }
 
+    /* Space saving heuristic.  Start filling from the left */
+    if (Py_SIZE(deque) == 0) {
+        assert(deque->leftblock == deque->rightblock);
+        assert(deque->leftindex == deque->rightindex+1);
+        deque->leftindex = 1;
+        deque->rightindex = 0;
+    }
+
     it = PyObject_GetIter(iterable);
     if (it == NULL)
         return NULL;
@@ -385,6 +393,14 @@ deque_extendleft(dequeobject *deque, PyObject *iterable)
         return result;
     }
 
+    /* Space saving heuristic.  Start filling from the right */
+    if (Py_SIZE(deque) == 0) {
+        assert(deque->leftblock == deque->rightblock);
+        assert(deque->leftindex == deque->rightindex+1);
+        deque->leftindex = BLOCKLEN - 1;
+        deque->rightindex = BLOCKLEN - 2;
+    }
+
     it = PyObject_GetIter(iterable);
     if (it == NULL)
         return NULL;