On 01/11/2010 15:10, R. David Murray wrote:
On Mon, 01 Nov 2010 14:26:19 -0000, Michael Foord<fuzzy...@voidspace.org.uk>
wrote:
On 01/11/2010 11:33, Antoine Pitrou wrote:
On Mon, 01 Nov 2010 02:55:35 +0000
Michael Foord<fuzzy...@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 =3D [{2,4}, {1,2}]
>>> b =3D 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]).
Well, as Antoine pointed out, Python's sorting algorithm is stable,
so that is in fact *not* an invariant:
x = ['abcd', 'foo'*50, 'foo'*50, 'dkke']
y = x[::-1]
[id(n) for n in sorted(x)]
[3073747680, 3073747904, 3073747624, 3073747512]
[id(n) for n in sorted(y)]
[3073747680, 3073747904, 3073747512, 3073747624]
Yes, == usually hides the fact that the *objects* are in a different
order, but obviously that doesn't apply to sets :)
Sorry, that should have been sorted(l) == sorted(l[::-1]) - which *is*
the case for your example above.
Michael
--
R. David Murray www.bitdance.com
--
http://www.voidspace.org.uk/
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