Author mark.dickinson
Recipients flox, mark.dickinson, rhettinger
Date 2009-12-17.20:36:49
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Message-id <1261082212.04.0.19696566525.issue7532@psf.upfronthosting.co.za>
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Hmm.  This doesn't look like something that's easy to fix without affecting existing correct code;  given 
that the behaviour has been around for a while (I'm not sure exactly how long), and that the issue is gone 
in 3.x, I suspect it may not be worth trying.

Analysis:  for classic classes, the ceval loop calls PySequence_GetSlice, which corrects the negative 
indices by adding the length of the sequence  (as described in the docs) and then calls the 
tp_as_sequence->sq_slice slot on the type.  That ends up calling instance_slice (in 
Objects/classobject.c), which discovers that the class doesn't implement __getslice__ and so passes the 
adjusted slice on to the __getitem__ method.

I'm not sure how this could be changed to get the correct behaviour:  PySequence_GetSlice would somehow 
need to know that it was going to end up calling __getitem__ rather than __getslice__.

Raymond, any thoughts?
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Date User Action Args
2009-12-17 20:36:52mark.dickinsonsetrecipients: + mark.dickinson, rhettinger, flox
2009-12-17 20:36:52mark.dickinsonsetmessageid: <1261082212.04.0.19696566525.issue7532@psf.upfronthosting.co.za>
2009-12-17 20:36:50mark.dickinsonlinkissue7532 messages
2009-12-17 20:36:49mark.dickinsoncreate