Backport r67478

Lib/test/list_tests.py

@@ -84,6 +84,8 @@ class CommonTest(seq_tests.CommonTest):
         self.assertRaises(StopIteration, r.next)
         self.assertEqual(list(reversed(self.type2test())),
                          self.type2test())
+        # Bug 3689: make sure list-reversed-iterator doesn't have __len__
+        self.assertRaises(TypeError, len, reversed([1,2,3]))
 
     def test_setitem(self):
         a = self.type2test([0, 1])

Misc/NEWS

@@ -16,6 +16,9 @@ Core and Builtins
   interpreter to abort ("Fatal Python error: Could not reset the stack!")
   instead of throwing a MemoryError.
 
+- Issue #3689: The list reversed iterator now supports __length_hint__
+  instead of __len__.  Behavior now matches other reversed iterators.
+
 - Issue #4367: Python would segfault during compiling when the unicodedata
   module couldn't be imported and \N escapes were present.
 

Objects/listobject.c

@@ -2911,11 +2911,11 @@ static PyObject *list_reversed(PyListObject *, PyObject *);
 static void listreviter_dealloc(listreviterobject *);
 static int listreviter_traverse(listreviterobject *, visitproc, void *);
 static PyObject *listreviter_next(listreviterobject *);
-static Py_ssize_t listreviter_len(listreviterobject *);
+static PyObject *listreviter_len(listreviterobject *);
 
-static PySequenceMethods listreviter_as_sequence = {
-	(lenfunc)listreviter_len,	/* sq_length */
-	0,				/* sq_concat */
+static PyMethodDef listreviter_methods[] = {
+	{"__length_hint__", (PyCFunction)listreviter_len, METH_NOARGS, length_hint_doc},
+ 	{NULL,		NULL}		/* sentinel */
 };
 
 PyTypeObject PyListRevIter_Type = {
@@ -2931,7 +2931,7 @@ PyTypeObject PyListRevIter_Type = {
 	0,					/* tp_compare */
 	0,					/* tp_repr */
 	0,					/* tp_as_number */
-	&listreviter_as_sequence,		/* tp_as_sequence */
+	0,					/* tp_as_sequence */
 	0,					/* tp_as_mapping */
 	0,					/* tp_hash */
 	0,					/* tp_call */
@@ -2947,6 +2947,7 @@ PyTypeObject PyListRevIter_Type = {
 	0,					/* tp_weaklistoffset */
 	PyObject_SelfIter,			/* tp_iter */
 	(iternextfunc)listreviter_next,		/* tp_iternext */
+	listreviter_methods,		/* tp_methods */
 	0,
 };
 
@@ -3002,11 +3003,11 @@ listreviter_next(listreviterobject *it)
 	return NULL;
 }
 
-static Py_ssize_t
+static PyObject *
 listreviter_len(listreviterobject *it)
 {
 	Py_ssize_t len = it->it_index + 1;
 	if (it->it_seq == NULL || PyList_GET_SIZE(it->it_seq) < len)
-		return 0;
-	return len;
+		len = 0;
+	return PyLong_FromSsize_t(len);
 }