On 01/11/2010 11:33, Antoine Pitrou wrote:
> On Mon, 01 Nov 2010 02:55:35 +0000
> Michael Foord<fuzzyman at voidspace.org.uk> wrote:
>> Having a more efficient 'slow-path' and moving to that by default would
>> fix it. The bug is only a duplicate of the bug in sorted - caused by the
>> fact that sets / frozensets can't be sorted in the standard Python way
>> (their less than comparison adheres to the set definition). This is
>> something that will probably surprise many Python developers:
>>
>> >>> a = [{2,4}, {1,2}]
>> >>> b = a[::-1]
>> >>> sorted(a)
>> [set([2, 4]), set([1, 2])]
>> >>> sorted(b)
>> [set([1, 2]), set([2, 4])]
>>
>> (Fixing the bug in sorted would fix assertItemsEqual ;-)
> How is this a bug? The sort algorithm is stable, which means the above
> behaviour is a feature.
Well, bug is the wrong word as it is obviously an intended feature (or
consequence of a feature). I still think, given the general behaviour of
Python sorting, that it is unexpected. It breaks what is usually an
invariant for sorting without an explicit key that sortable types
sorted(l) == sorted(l[::-1]).
There is however a note in the set documentation:
Since sets only define partial ordering (subset relationships), the
output of the list.sort() method is undefined for lists of sets.
> I see no easy way of eliminating the O(n*n) issue. Custom key functions
> can't work in all cases.
>
Right. Special casing sets and frozensets would be one (particularly
inelegant) way however.
All the best,
Michael
> Regards
>
> Antoine.
>
>
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