Non-deterministic set ordering

[Rob Cliffe]

On 16/05/2022 04:13, Dan Stromberg wrote:
>
> On Sun, May 15, 2022 at 8:01 PM Rob Cliffe via Python-list 
> <python-list at python.org> wrote:
>
>     I was shocked to discover that when repeatedly running the following
>     program (condensed from a "real" program) under Python 3.8.3
>
>     for p in { ('x','y'), ('y','x') }:
>          print(p)
>
>     the output was sometimes
>
>     ('y', 'x')
>     ('x', 'y')
>
>     and sometimes
>
>     ('x', 'y')
>     ('y', 'x')
>
>     Can anyone explain why running identical code should result in
>     traversing a set in a different order?
>
>
> Sets are defined as unordered so that they can be hashed internally to 
> give O(1) operations for many tasks.
>
> It wouldn't be unreasonable for sets to use a fixed-by-arbitrary 
> ordering for a given group of set operations, but being unpredictable 
> deters developers from mistakenly assuming they are ordered.
>
> If you need order, you should use a tuple, list, or something like 
> https://grantjenks.com/docs/sortedcontainers/sortedset.html
Thanks, I can work round this behaviour.
But I'm curious: where does the variability come from?  Is it deliberate 
(as your answer seems to imply)?  AFAIK the same code within the *same 
run* of a program does produce identical results.
Best wishes
Rob Cliffe