[Python-ideas] Fast sum() for non-numbers

[Terry Reedy]

On 7/5/2013 12:22 PM, Ron Adam wrote:
>
>
> On 07/03/2013 07:43 AM, Steven D'Aprano wrote:
>>
>> I'm not sure that sum() is the Obvious Way to concatenate lists, and I
>> don't think that concatenating many lists is a common thing to do.

Agree here.

>> Traditionally, sum() works only on numbers, and I think we wouldn't be
>> having this discussion if Python used & for concatenation instead of
>> +.

Since addition of numbers represented in base-1 or tally notation is the 
same as sequence concatenation, I think '+' is the right choice.

In my experience of American English, normal people 'add' lists (though 
perhaps as sets, with duplicate  removal), not 'concatenate' them. And 
while they may 'add' together multiple lists, they would not likely 
'sum' them unless they are lists of numbers.

When Alex said that it was not possible to determine if the start value 
is a number, he was talking in the context of old style classes where 
the type of every user class was 'Class' and the type of every user 
instance was 'Instance' (or something like that). In Python 3, with 
ABCs, isinstance(start, Number) would solve the problem as long as the 
requirement were documented.

-- 
Terry Jan Reedy