sum() requires number, not simply __add__

[Chris Angelico]

On Fri, Feb 24, 2012 at 8:41 AM, Arnaud Delobelle <arnodel at gmail.com> wrote:
> _sentinel = object()
>
> def sum(iterable, start=_sentinel):
>    if start is _sentinel:
>
> del _sentinel

Somewhat off-topic: Doesn't the if statement there do a lookup for a
global, which would mean that 'del _sentinel' will cause it to fail?
Or have I missed something here?

ChrisA