sum for sequences?

Steven D'Aprano steven at
Mon Mar 29 04:45:32 CEST 2010

On Sun, 28 Mar 2010 18:56:26 +0200, Alf P. Steinbach wrote:

> From a more practical point of view, the sum efficiency could be
> improved by doing the first addition using '+' and the rest using '+=',
> without changing the behavior.

But that would change the behaviour. The __iadd__ method is not the same 
as the __add__ method, and you can't guarantee that they will behave the 
same -- or even similarly.

And what about tuples? And subclasses of list/tuples? How many different 
types need to be optimized?

In practical terms, does anyone actually ever use sum on more than a 
handful of lists? I don't believe this is more than a hypothetical 

The primary use case for sum is adding numbers when floating point 
accuracy is not critical. If you need float accuracy, use math.fsum. If 
you need to add more than a handful of small lists, don't use sum: just 
calling extend in a loop is probably fast enough, or use itertools.chain. 
Trying to turn sum into an all-singing all-dancing function optimised for 
an ever-increasing number of objects is a fool's errand. The 
implementation of sum in C is already complex enough: 95 lines, excluding 
comments, blanks and braces, for something which is a lightweight 
function with very simple semantics.

But if anyone wants to submit a patch to the bug tracker, go right ahead. 
Without a patch though, I'd say that Python-Dev will consider this a non-


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