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

[jepler at unpythonic.net]

One reason might be Practicality.  The zip() versions handily beat the
listcomp versions on a 10kitem dict. (python2.4)

$ timeit.py -s 'd = dict.fromkeys(range(10000))' '[(v, k) for (k, v) in d.iteritems()]'
100 loops, best of 3: 5.05 msec per loop
$ timeit.py -s 'd = dict.fromkeys(range(10000))' '[(v, k) for (k, v) in d.items()]'
100 loops, best of 3: 7.14 msec per loop
$ timeit.py -s 'd = dict.fromkeys(range(10000))' 'zip(d.itervalues(), d.iterkeys())'
100 loops, best of 3: 3.13 msec per loop
$ timeit.py -s 'd = dict.fromkeys(range(10000))' 'zip(d.values(), d.keys())'
100 loops, best of 3: 3.28 msec per loop

For comparison,
$ timeit.py -s 'd = dict.fromkeys(range(10000))' 'd.items()'
100 loops, best of 3: 2.19 msec per loop

Maybe there are also other reasons to promise this property that I'm not
aware of.  Certainly it seems like this property is useful and not hard
to provide for "non-perverse" implementations, much like the
now-documented stability of sort().

Jeff
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