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@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
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