Non-deterministic set ordering

[Rob Cliffe]

Thanks, Paul.  Question answered!
Rob Cliffe

On 16/05/2022 04:36, Paul Bryan wrote:
> 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
>