Non-deterministic set ordering

[Paul Bryan]

This may explain it:
https://stackoverflow.com/questions/27522626/hash-function-in-python-3-3-returns-different-results-between-sessions

On Mon, 2022-05-16 at 04:20 +0100, Rob Cliffe via Python-list wrote:
> 
> 
> 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