[Python-Dev] set iteration order

[Raymond Hettinger]

On Feb 26, 2011, at 4:09 PM, Antoine Pitrou wrote:

> On Sat, 26 Feb 2011 10:09:33 +0100
> Hagen Fürstenau <hagen at zhuliguan.net> wrote:
>> 
>> I just hunted down a change in behaviour between Python 3.1 and 3.2 to
>> possibly changed iteration order of sets due to the optimization in
>> issue #8685. Of course, this order shouldn't be relied on in the first
>> place, but the side effect of the optimization might be worth mentioning
>> in "What's new", maybe also pointing out that the old behaviour can be
>> simulated with {x for x in a if x not in b} in place of "a-b".
> 
> I'm against such a mention. It would give the impression that we
> support some semblance of reproduceability in iteration order, which is
> not true. If you use sets or dicts, you must deal with the fact that
> the iteration order will be totally random from your (the programmer's)
> POV.

I concur with Antoine.

Also, it wasn't the iteration order that changed; sets still iterate in the same order.
What changed was the algorithm for creating a new set using a set difference operation.

Raymond