[Python-ideas] Why does `sum` use a default for the `start` parameter?

Vitor Bosshard algorias at gmail.com
Sat Dec 5 19:04:42 CET 2009

2009/12/5 Georg Brandl <g.brandl at gmx.net>:
> Vitor Bosshard schrieb:
>> 2009/12/5 George Sakkis <george.sakkis at gmail.com>:
>>> On Sat, Dec 5, 2009 at 6:45 PM, Andre Engels <andreengels at gmail.com> wrote:
>>>> On Sat, Dec 5, 2009 at 12:55 PM, Ram Rachum <cool-rr at cool-rr.com> wrote:
>>>>> I noticed that `sum` tries to add zero to your iterable. Why? Why not just skip
>>>>> adding any start value if none is specified?
>>>>> This current behavior is preventing me from using `sum` to add up a bunch of non-
>>>>> number objects.
>>>> In your proposed implementation, sum([]) would be undefined.
>>> Which would make it consistent with min/max.
>> And in that case the special string handling could also be dropped?
>>>>> sum(["a","b"], "start")
>> Traceback (most recent call last):
>>   File "<pyshell#0>", line 1, in <module>
>>     sum(["a","b"], "start")
>> TypeError: sum() can't sum strings [use ''.join(seq) instead]
>> This behaviour is quite bothersome. Sum can handle arbitrary objects
>> in theory (as long as they define the correct special methods, etc.),
>> but it gratuitously raises an exception on strings.
> This seems to be an instance where the "practicality" Zen rule beats the
> "special cases" rule :)

It might be more accurate to say "hand-holding" instead of
practicality (and it doesn't even catch all errors it's meant to). I'm
not so sure that's special enough ;-)


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