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

[Ronald Oussoren]

On 9 Jul, 2013, at 15:54, Sergey <sergemp at mail.ru> wrote:

> On Jul 9, 2013 Steven D'Aprano write:
> 
>>> Well... Yes, I can! I can't make __iadd__ faster, because tuple has
>>> no __iadd__, however I can make a faster __add__.
>> 
>> And how do you expect to do that? Tuples are immutable, you have
>> to create a new tuple. So when adding a sequence of N tuples
>> together, you end up making and throwing away N-1 intermediate
>> results.
> 
> For example, rewrite tuple to internally store its values in a list,
> and have `localcount` variable saying how many items from that list
> belong to this tuple. Then __add__ could extend that list and reuse
> it for new tuple.

That's not going to happen, not only breaks that backward compatibility
for users for the C API, it has nasty side effects and is incorrect.

Nasty side effect:

a = (1,)
b = (2,) * 1000
c = a + b
del b
del c

With the internal list 'a' keeps alive the extra storage used for 'c'.

Incorrect:

a = (1,)
b = a + (2,)
c = a + (3,)

Now 'b' and 'c' can't possibly both share storage with 'a'.

Ronald