Dictionary .keys() and .values() should return a set [withPython3000 in mind]

[Delaney, Timothy (Tim)]

Paul Rubin wrote:

> "Delaney, Timothy (Tim)" <tdelaney at avaya.com> writes:
>> The eventual consensus was that keys(), etc should return views on
>> the dictionary. These views would be re-iterable and will have
>> basically the same behaviour as the lists returned from keys(), etc.
>> However, such a view could have O(1) __contains__ behaviour, and
>> would not incur the overhead of creating a list and populating it -
>> they would be backed by the dictionary. Of course, this introduces
>> the issue of concurrent modification, but you can always get an
>> independent copy by calling list(dict.keys()).
> 
> Wait a sec, you're saying
> 
>   k0 = d.keys()
>   d['whee'] = 'parrot'    # this can change k0???
> 
> That sounds broken.

That's the problem with views. You either take a snapshot of the current
state, or you deal with the backing store changing. Java's solution is
to raise a ConcurrentModificationException when it determines this has
happened (with lots of weasel words about no guarantees).

If you want an independent data set, you have to take a snapshot. For
the above, that's doing:

    k0 = list(d.keys())

or

    k0 = set(d.keys())

depending on what behaviour you want.

Tim Delaney