From 0e0e21530821e7b3218fca3fa7a286ed6088eceb Mon Sep 17 00:00:00 2001 From: Mark Dickinson Date: Mon, 26 Oct 2009 14:18:44 +0000 Subject: [PATCH] Warn against replacing PyNumber_Add with PyNumber_InPlaceAdd in sum --- Python/bltinmodule.c | 9 +++++++++ 1 file changed, 9 insertions(+) diff --git a/Python/bltinmodule.c b/Python/bltinmodule.c index 1f25d5d2087..c0607706dcf 100644 --- a/Python/bltinmodule.c +++ b/Python/bltinmodule.c @@ -2350,6 +2350,15 @@ builtin_sum(PyObject *self, PyObject *args) } 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);