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@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 -- https://mail.python.org/mailman/listinfo/python-list