This isn't the first time around in this discussion of (abusing) 'sum()'. Actually, I discussed the last round in my keynote at PyCon UK in 2013. In particular, I actually discussed there already how Chris' latest version of 'sum()' breaks *existing* and *widespread* code.

My slides are at:

http://gnosis.cx/pycon-uk-2013/Keynote-Ideas.pdf

Look at slides 24-26. The TL;DR version: numpy arrays give different meanings to __add__() and __iadd__(). Chris' version breaks every program ever written that uses numpy.

On Wed, Aug 13, 2014 at 11:05 AM, Chris Kaynor <ckaynor@zindagigames.com> wrote:

On Wed, Aug 13, 2014 at 10:46 AM, Steven D'Aprano <steve@pearwood.info> wrote:

That's another semantic change: sum() currently accepts iterators
(although not *infinite* iterators). This revision has now broken tens
of thousands of applications.

okay, how about:

def sum(items, start=0):
first = True

for item in items:
if first:
start = start + item
else:
start += item

return start

I believe that has the same effect as my other two, but only adds the requirement that "value += item" behaves the same as "value = value + item".

Chris

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