Warn against replacing PyNumber_Add with PyNumber_InPlaceAdd in sum
diff --git a/Python/bltinmodule.c b/Python/bltinmodule.c
index 1f25d5d..c060770 100644
--- a/Python/bltinmodule.c
+++ b/Python/bltinmodule.c
@@ -2350,6 +2350,15 @@
}
break;
}
+ /* It's tempting to use PyNumber_InPlaceAdd instead of
+ PyNumber_Add here, to avoid quadratic running time
+ when doing 'sum(list_of_lists, [])'. However, this
+ would produce a change in behaviour: a snippet like
+
+ empty = []
+ sum([[x] for x in range(10)], empty)
+
+ would change the value of empty. */
temp = PyNumber_Add(result, item);
Py_DECREF(result);
Py_DECREF(item);