Tidied up the implementations of reversed (including the custom ones

for xrange and list objects).

* list.__reversed__ now checks the length of the sequence object before
  calling PyList_GET_ITEM() because the mutable could have changed length.

* all three implementations are now tranparent with respect to length and
  maintain the invariant len(it) == len(list(it)) even when the underlying
  sequence mutates.

* __builtin__.reversed() now frees the underlying sequence as soon
  as the iterator is exhausted.

* the code paths were rearranged so that the most common paths
  do not require a jump.

1 files changed, 20 insertions, 16 deletions

diff --git a/Objects/enumobject.c b/Objects/enumobject.c
index 1d13123..28719a9 100644
--- a/Objects/enumobject.c
+++ b/Objects/enumobject.c
@@ -217,23 +217,21 @@ static PyObject *
 reversed_next(reversedobject *ro)
 {
 	PyObject *item;
+	long index = ro->index;
 
-	if (ro->index < 0)
-		return NULL;
-
-	assert(PySequence_Check(ro->seq));
-	item = PySequence_GetItem(ro->seq, ro->index);
-	if (item == NULL)
-		return NULL;
-
-	ro->index--;
-	return item;
-}
-
-static int
-reversed_len(reversedobject *ro)
-{
-	return PyObject_Size(ro->seq);
+	if (index >= 0) {
+		item = PySequence_GetItem(ro->seq, index);
+		if (item != NULL) {
+			ro->index--;
+			return item;
+		}
+	}
+	ro->index = -1;
+	if (ro->seq != NULL) {
+		Py_DECREF(ro->seq);
+		ro->seq = NULL;
+	}
+	return NULL;
 }
 
 PyDoc_STRVAR(reversed_doc,
@@ -241,6 +239,12 @@ PyDoc_STRVAR(reversed_doc,
 "\n"
 "Return a reverse iterator");
 
+static int
+reversed_len(reversedobject *ro)
+{
+	return ro->index + 1;
+}
+
 static PySequenceMethods reversed_as_sequence = {
 	(inquiry)reversed_len,		/* sq_length */
 	0,				/* sq_concat */