> One way to think about is that adding indexing to the views would NOT make them Sequences, it would simply provide that one feature
As you can see in an e-mail from me in another thread yesterday - `__eq__`, oddly enough, is not currently a Sequence method, if we take Sequences to be what is collections.abc.Sequence interface. (In that thread e-mail I am actually querying if there would be no drawbacks in doing that, though) On Tue, 30 Jun 2020 at 15:10, Christopher Barker <python...@gmail.com> wrote: > On Tue, Jun 30, 2020 at 5:35 AM Steven D'Aprano <st...@pearwood.info> > wrote: > >> On Mon, Jun 29, 2020 at 11:02:54PM -0700, Christopher Barker wrote: >> > > >> > Or is it a set because it can’t be indexed? If I have the history right, >> > dict.items was first implemented with the “old” dict implementation, >> which >> > did not preserve order, but did provide O(1) access, so making the dict >> > views set-like was easy, and making them Sequences was impossible. >> >> No, making them set-like was intentional. >> >> The history goes something like this: >> >> In Python 1, and early Python 2, dict.keys(), .values() and .items() >> each returned lists. Some time by 2.4 (possibly earlier) dicts grew >> additional methods returning iterators: >> >> dict.iterkeys() .itervalues() .iteritems() >> >> As part of the migration from Python 2 to 3, it was planned to change >> them to set-like views: >> >> https://www.python.org/dev/peps/pep-3106/ >> >> so as an aid to migration, these set-like views were added to 2.7: >> >> dict.viewkeys() .viewvalues() .viewitems() >> >> and finally in 3.x, the list and iterator versions were all dropped and >> the views were renamed. >> >> So there was already a sequence version of items etc, and the set-like >> views were explicitly intended to be set-like. >> > > Thanks for the details, but I just re-read the PEP, and I'm not sure my > hand wavy version is wrong: > > > there was already a sequence version of items etc > > Well, yes, but they explicitly created list copies -- there was no > sequence-like version that was a view. > > The first improvement on that was the iter* methods, and then (inspired by > JAVA) it was determined that we could have view objects that could provide > efficient iterators, and also set-like behavior. But I don't see any > comments in the PEP about why indexing was rejected, but it's implicit in > the fact that dicts were explicitly unordered at the time that PEP was > written. > > Now that dicts do preserve order, and there is an implementation that can > provide somewhat efficient indexing ( not O(1), but yes O(N) with a small > constant) there is the opportunity to add a new feature. > > It does bring up a question: if the view objects support indexing, it > would be a bit odd for two dict view objects to be equal if they have the > same elements, but not the same order, but that would be required to remain > consistent with the way dicts currently are (and will remain) One way to > think about is that adding indexing to the views would NOT make them > Sequences, it would simply provide that one feature. > > So how useful is that feature? I recall a couple (actually, essentailly > one) which is selecting an arbitrary or random item from a dict, i.e. being > able to pass a dict view to random.choice() > > As it happens. I've had that use case, but if that's the only one folks > can think of, then maybe not particularly worthwhile. > > Final note about performance: yes, we don't want to encourage folks to use > not-great-performance approaches, but the alternative to the O(N) with > small constant performance being proposed here is to make a Sequence out of > the views, and then index into them, which is O(N) with a larger constant > and more memory use. > > -CHB > > > > > > > > > > > > > > > > >> >> -- >> Steven >> _______________________________________________ >> 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/XT4T7OIEAY7YQZR6K2ODOQZRXT24YIXY/ >> Code of Conduct: http://python.org/psf/codeofconduct/ >> > > > -- > Christopher Barker, PhD > > Python Language Consulting > - Teaching > - Scientific Software Development > - Desktop GUI and Web Development > - wxPython, numpy, scipy, Cython > _______________________________________________ > 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/KHXGUVNGQY6RJSDWKFXBK4G5UQAJ75U7/ > Code of Conduct: http://python.org/psf/codeofconduct/ >
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