[Python-Dev] Comparison and (user) confusion (original) (raw)

François Pinard pinard at iro.umontreal.ca
Thu Jul 1 19:37:16 CEST 2004

Hi, people.

Yesterday, as a user, I made a little bug which I quickly resolved, but over which I could have spent some time, having been younger to Python.

The problem appeared when I sub-classed list', and used a cmp' method in my sub-type to effect a specialised sort. The sort did not behave as expected, as the list' type provides rich comparisons, and that sort', finding __lt__', merely ignored my own cmp'.

I wonder if Python could not have helped better here, by diagnosing a possible comparison confusion while creating the sub-class. I do not know much know Python internals, but if C slots happen to be used for comparison methods, it might not be so unreasonable to expect such confusion to be detected whenever those slots are filled.

I presume here, maybe wrongly, that it does not make good sense mixing rich and non-rich comparisons in the same class. Just a thought. :-)

P.S. - Of course, replacing __cmp__' by lt' solved my problem.

P.P.S. - This is Python 2.3.3 on Linux, in case it matters.

-- François Pinard http://www.iro.umontreal.ca/~pinard

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