On Mon, Nov 28, 2022 at 5:38 PM Steven D'Aprano <st...@pearwood.info> wrote:
> On Mon, Nov 28, 2022 at 11:13:34PM +0000, Oscar Benjamin wrote: > > On Mon, 28 Nov 2022 at 22:56, Brett Cannon <br...@python.org> wrote: > > > > That's actually by design. Sets are not meant to be deterministic > > > conceptually as they are essentially a bag of stuff. If you want > > > deterministic ordering you should convert it to a list and sort the > > > list. > > > > What does "sets are not meant to be deterministic" even mean? > > I'm not Brett, so I'm not answering for him, but many people (sometimes > including me) misuse "not deterministic" to refer to set order and the > old dict order. Yep, that's what I mean. And my comment about dicts earlier is old knowledge from before we made dict iteration deterministic based on insertion order since I'm been doing this for too long. 😉 -Brett > Of course set order is deterministic, it is determined > by the combination of the set implementation, the hashing algorithm, and > the history of the set -- all the items that have ever appeared in the > set (both those removed and those that remain). > > Set order is deterministic in the same way that roulette wheels are > deterministic. > > We don't have a good term for this: the items don't appear in random > order. But it is not predictable either, except in very special cases. > "Arbitrary" is not right either, since that implies we can easily choose > whatever set order we want. > > I think that physicists call this "deterministic chaos" > > > https://www.quora.com/Theoretical-Physics-What-is-deterministic-but-unpredictable > > (sorry for the quora link) so I guess we might say that set iteration > order is "deterministically chaotic" if you want to be precise, but life > is too short for that level of pedantry (and that's coming from me, a > Grade A Pedant *wink*) so people often describe it as random, arbitrary, > or non-deterministic when it's none of those things :-). > > Maybe we could call set order "pseudo-random". > > Getting back to the design part, I think that what Brett is trying to > get across is not that the chaotic set order is in and of itself a > requirement, but that given the other requirements, chaotic set order is > currently considered a necessary condition. > > As I understand it, we could make sets ordered, but only at the cost of > space (much more memory) or time (slower) or both. > > I am sure that Guido is correct that **if** somebody comes up with a > fast, efficient ordered set implementation that doesn't perform worse > than the current implementation, we will happily swap to giving sets a > predictable order, as we did with dicts. (Practicality beats purity -- > even if sets are *philosophically* unordered, preserving input order is > too useful to give up unless we gain something in return.) > > But I don't think it is fair or kind to call Brett's argument FUD. At > the very least it is uncharitable interpretation of Brett's position. > > > It would be useful to have a straight-forward way to sort a set into a > > deterministic ordering but no such feature exists after the Py3K > > changes (sorted used to do this in Python 2.x). > > `sorted()` works fine on homogeneous sets. It is only heterogeneous sets > that are a problem, and in practice, that usually means None mixed in > with some other type. > > > -- > Steve > _______________________________________________ > Python-Dev mailing list -- python-dev@python.org > To unsubscribe send an email to python-dev-le...@python.org > https://mail.python.org/mailman3/lists/python-dev.python.org/ > Message archived at > https://mail.python.org/archives/list/python-dev@python.org/message/UAVNSMS3XFAQQ6ZRD27IXPTWZFCHP6M4/ > Code of Conduct: http://python.org/psf/codeofconduct/ >
_______________________________________________ Python-Dev mailing list -- python-dev@python.org To unsubscribe send an email to python-dev-le...@python.org https://mail.python.org/mailman3/lists/python-dev.python.org/ Message archived at https://mail.python.org/archives/list/python-dev@python.org/message/ZUACMM6HDUOG4VIAPAK7W5Q6WQRRLI2U/ Code of Conduct: http://python.org/psf/codeofconduct/
