On Fri, Jul 31, 2020 at 8:44 PM Guido van Rossum <gu...@python.org> wrote:
> This is not great news. A solution could be to make dict.ordered() force > compaction -- if there are no deleted keys this would be a no-op. We'd have > to be able to tell in constant time whether this is the case. But yeah, > this would be a dangerous thing in the hands of folks below wizard level. > Another solution could be to make dict.ordered() fail if there are deleted > keys. But that's not a great experience either. > > All in all I retract this idea. > > On Fri, Jul 31, 2020 at 5:31 PM Stestagg <stest...@gmail.com> wrote: > >> On Sat, 1 Aug 2020 at 00:32, Guido van Rossum <gu...@python.org> wrote: >> >>> If true this would be a huge argument against adding indexing and >>> slicing (other than the special case starting with 0). However, I don't >>> think it's true. The dict implementation (again, starting in 3.6) actually >>> stores the list of keys in a compact array, in the insertion order. The >>> hash table stores indices into this array. Moreover, in order to implement >>> the ordered property, you pretty much have to do this (since the hash table >>> *definitely* isn't going to be ordered :-). So indexing and slicing into >>> this array would seem to be possible, unless I am missing something. I'm >>> CC"ing Inada Naoki because it's his code -- maybe he can enlighten us. >>> >> >> I’m not Inada Naoki :). So happy to be corrected, but I looked into this >> quite closely. >> >> The dict keys is compact only *until* you delete an item, at which point, >> a hole is left in the array, which defeats direct lookups (either by >> indexing or via a view). Order preservation is still kept as code that >> iterates over dictionaries knows to just skip the holes. >> >> I’m unsure when/if compaction is called however. So there may be a >> technique for forcing the keys to be compacted again. This would cause >> very unpredictable performance in the case of large dictionaries (a main >> use-case here) >> >> Steve >> >> >>> _______________________________________________ >> Python-ideas mailing list -- python-ideas@python.org >> To unsubscribe send an email to python-ideas-le...@python.org >> https://mail.python.org/mailman3/lists/python-ideas.python.org/ >> Message archived at >> https://mail.python.org/archives/list/python-ideas@python.org/message/CWQMLXNOEPCXUXPQSTUOI5VUF4V7R36B/ >> Code of Conduct: http://python.org/psf/codeofconduct/ >> > > > -- > --Guido van Rossum (python.org/~guido) > *Pronouns: he/him **(why is my pronoun here?)* > <http://feministing.com/2015/02/03/how-using-they-as-a-singular-pronoun-can-change-the-world/> > _______________________________________________ > Python-ideas mailing list -- python-ideas@python.org > To unsubscribe send an email to python-ideas-le...@python.org > https://mail.python.org/mailman3/lists/python-ideas.python.org/ > Message archived at > https://mail.python.org/archives/list/python-ideas@python.org/message/JY2ZJTOE4VHC36VWTMZY4QM7U2NUCYT5/ > Code of Conduct: http://python.org/psf/codeofconduct/ >
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