On Wed, Jul 24, 2019 at 05:30:19PM -0000, Brett Cannon wrote: > When I saw this I thought, "it should be like `set(d1.values()) == > set(d2.values())`", but has been pointed out there's no guarantee that > all values will be hashable.
The hashability requirement for sets is, in a sense, an implementation detail. It might be a requirement for sets in Python the language, but its not a requirement for abstract "sets of values". E.g. Java includes a standard TreeSet which doesn't require hashability https://docs.oracle.com/javase/7/docs/api/java/util/TreeSet.html In this case, they need to be multisets, since {'a': 1, 'b': 2, 'c': 1}.values() != {'a': 1, 'b': 2, 'c': 2}.values() > After that I have no expectation since order isn't guaranteed. > > I think this is one of those cases where it's superficially surprising > when you don't think about all the ramifications, but once you > understand the complexity of the problem then it becomes more clear > that it isn't straight-forward. Nobody said it was straight-forward, particularly if we want guaranteed efficient comparisons in both time and space at the same time. Brett, I feel that you are dismissing this thread as "not thinking through the ramifications" without reading it through, because I'm pretty sure that we have thought through the ramifications in a lot more detail than your dismissal justifies. Let's start with the minimal change we have suggested: that two views should be considered equal if they both belong to the same dict. assert d.values() == d.values() Currently that assertion fails. Should it? Putting aside the convenience of "do nothing, just inherit the object.__eq__ behaviour" do you think that the current behaviour is *correct*? (By correct I mean in the technical sense that if we were writing a functional spec for views, we would actively desire two views of the same dict to be unequal.) > To me a doc update for dict.values() stating that the iterator can't > be compared and a brief mention as to why would be the best solution > for this. We're not talking about comparing iterators. We're talking about comparing views into a dict. That's a difference that makes all the difference. -- Steven _______________________________________________ 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/UPFBDHOHFSFLDF5KO3QZE6RH3C2GXE6B/
