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Date: Thu, 23 Feb 2012 21:41:28 +0000
Subject: Re: sum() requires number, not simply __add__
From: Arnaud Delobelle <arnodel@gmail.com>
To: Buck Golemon <buck@yelp.com>
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Xref: x330-a1.tempe.blueboxinc.net comp.lang.python:20753

On 23 February 2012 21:23, Buck Golemon <buck@yelp.com> wrote:
> def sum(values,
> base=3D0):
> =C2=A0 =C2=A0 =C2=A0values =3D
> iter(values)
>
> =C2=A0 =C2=A0 =C2=A0try:
> =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0result =3D values.next()
> =C2=A0 =C2=A0 =C2=A0except StopIteration:
> =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0return base
>
> =C2=A0 =C2=A0 =C2=A0for value in values:
> =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0result +=3D value
> =C2=A0 =C2=A0 =C2=A0return result

This is definitely not backward compatible.  To get something that has
a better chance of working with existing code, try this (untested):

_sentinel =3D object()

def sum(iterable, start=3D_sentinel):
    if start is _sentinel:
        iterable =3D iter(iterable)
        try:
            start =3D iterable.next()
        except StopIteration:
            return 0
    for x in iterable:
        start +=3D x
    return start

del _sentinel

--=20
Arnaud