Non-deterministic set ordering

[Dan Stromberg]

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