From 5b44cbe6d8f9c07f6ec0da1134dc3b49ee81a4e3 Mon Sep 17 00:00:00 2001
From: Raymond Hettinger <python@rcn.com>
Date: Mon, 8 Jan 2007 18:05:53 +0000
Subject: [PATCH] Fix zero-length corner case for iterating over a mutating
 deque.

---
 Lib/test/test_deque.py      | 6 ++++++
 Misc/NEWS                   | 3 +++
 Modules/collectionsmodule.c | 5 ++---
 3 files changed, 11 insertions(+), 3 deletions(-)

diff --git a/Lib/test/test_deque.py b/Lib/test/test_deque.py
index a5629229936..35e15365329 100644
--- a/Lib/test/test_deque.py
+++ b/Lib/test/test_deque.py
@@ -396,6 +396,12 @@ class TestVariousIteratorArgs(unittest.TestCase):
         d.pop()
         self.assertRaises(RuntimeError, it.next)
 
+    def test_runtime_error_on_empty_deque(self):
+        d = deque()
+        it = iter(d)
+        d.append(10)
+        self.assertRaises(RuntimeError, it.next)
+
 class Deque(deque):
     pass
 
diff --git a/Misc/NEWS b/Misc/NEWS
index 76ed61e8354..4df89dade95 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -129,6 +129,9 @@ Extension Modules
 
 - Added support for linking the bsddb module against BerkeleyDB 4.5.x.
 
+- Modifying an empty deque during iteration now raises RuntimeError
+  instead of StopIteration.
+
 
 Library
 -------
diff --git a/Modules/collectionsmodule.c b/Modules/collectionsmodule.c
index c1bd732b322..a0570cde373 100644
--- a/Modules/collectionsmodule.c
+++ b/Modules/collectionsmodule.c
@@ -911,15 +911,14 @@ dequeiter_next(dequeiterobject *it)
 {
 	PyObject *item;
 
-	if (it->counter == 0)
-		return NULL;
-
 	if (it->deque->state != it->state) {
 		it->counter = 0;
 		PyErr_SetString(PyExc_RuntimeError,
 				"deque mutated during iteration");
 		return NULL;
 	}
+	if (it->counter == 0)
+		return NULL;        
 	assert (!(it->b == it->deque->rightblock &&
 		  it->index > it->deque->rightindex));