Why is dictionary.keys() a list and not a set?

[bonono at gmail.com]

Mike Meyer wrote:
> "bonono at gmail.com" <bonono at gmail.com> writes:
> > Christoph Zwerschke wrote:
> >> jepler at unpythonic.net schrieb:
> >> > You can already get a set from a dictionary's keys in an efficient manner:
> >> >>>>l = dict.fromkeys(range(10))
> >> >>>>set(l)
> >> > Set([0, 1, 2, 3, 4, 5, 6, 7, 8, 9])
> >> Good point. I expected that set(l) = set(l.items()) and not
> >> set(l.keys()), but the latter would not work with mutable values. See
> >> discussion with Martin.
> > puzzled. items() return tuples which I believe can be element of set ?
> > Or I misread you ?
>
> Not all tuples can be elements of a set. Elements of a set have to be
> hashable. Tuples compute their hash by hashing their contents. If
> their contents aren't hashable, the tuple isn't hashable, and hence
> can't be an element of a set. If the values in the dictionary aren't
> hashable, then the tuples returned by items() won't be hashable
> either, and hence can't be elements of a set.
>
A related issue, from the doc :

Set elements are like dictionary keys; they need to define both
__hash__ and __eq__ methods.

and dir(any_tuple) would show __hash__ and __eq__, would that be a bit
confusing as even though tuple has these two properties(or whatever
terms it should be called), it really depends what it contains ?

Or in other words, tuple is hashable if and only if every object it
contains is also hashable ?

If that is the case, would it be better to correct the doc to say such
thing ?