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/>
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