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From: Ian Kelly <ian.g.kelly@gmail.com>
Date: Thu, 23 Feb 2012 15:00:07 -0700
Subject: Re: sum() requires number, not simply __add__
To: Chris Angelico <rosuav@gmail.com>
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Xref: x330-a1.tempe.blueboxinc.net comp.lang.python:20758

On Thu, Feb 23, 2012 at 2:53 PM, Chris Angelico <rosuav@gmail.com> wrote:
> On Fri, Feb 24, 2012 at 8:41 AM, Arnaud Delobelle <arnodel@gmail.com> wro=
te:
>> _sentinel =3D object()
>>
>> def sum(iterable, start=3D_sentinel):
>> =A0 =A0if 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?

I believe you're correct.  If you really want to delete the _sentinel
reference though, you could do:

def sum(iterable, start=3Dobject()):
    if start is sum.func_defaults[0]:
        ...

Cheers,
Ian