[Python-Dev] Stable sort and partial order (original) (raw)

Michael Foord fuzzyman at voidspace.org.uk
Mon Nov 1 17:26:35 CET 2010

On 01/11/2010 16:23, Nick Coghlan wrote:

On Tue, Nov 2, 2010 at 1:33 AM, R. David Murray<rdmurray at bitdance.com> wrote:

Or, to put it another way, if there is a bug here it would be in set, not sorted. Put me in the "it's not a bug, it's a feature" camp. Providing a "elements equal" check that doesn't rely on LT providing a total ordering is a non-trivial exercise. Looking at assertItemsEqual, I'd be inclined to insert a check that falls back to the "unorderablelistdifference" approach in the case where "expected != sorted(reversed(expected))"

If that is sufficient then it would be a nice way of keeping the fast path.

(I'm not arguing that Antoine and R. David aren't correct in what they're saying about set ordering - I'm just saying that I was surprised and bet I'm not the only one. Bit of a dead end discussion. :-)

Michael

(only need to check the one, since if the expected values are totally ordered, while the actual values are not, this should show up when comparing the elements). It slows down the fast path a bit, but the updated function should at least handle partial orderings more correctly than it does now.

Cheers, Nick. P.S. Late night post, so I may be missing something obvious...

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