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

[Christoph Zwerschke]

Martin v. Löwis schrieb:

>> As Mike has written in his last posting, you could easily "fix" that 
>> by tweaking the equality relation as well. So technically speaking, 
>> Mike is probably right.
> 
> No. If you define both __hash__ and __eq__ consistently, then __hash__
> would meet the specification. As posted in the example, __hash__ does
> not meet the specification, contrary to Mike's claim that it does.

Ok, the example was incomplete, but in principle, this could be fixed by 
adding a tweaked __eq__ method. So for completeness sake:

class mylist2(list):
     def __hash__(self): return id(self)
     def __eq__(self, other): return self is other
     def __ne__(self, other): return self is not other

list = mylist2

a = list([1])
b = list([1])

print a, b, a == b, a != b

> If you have a=[1] and b=[1], then *always* a==b; you cannot
> change the semantics of list displays.

Surely, since you can't change the methods of built-in types. But you 
can create your own local list displays with a tweaked semantics as in 
the above example.

Anyway, the original question was: Are mylist1 and mylist2 (as above) to 
be considered "hashable" types or not? I think "technically" speaking, 
they are, and the "technical" definition is the only one that counts.

-- Christoph