[issue7532] Extended slicing with classic class behaves strangely

[Mark Dickinson]

Mark Dickinson <dickinsm at gmail.com> added the comment:

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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nosy: +mark.dickinson, rhettinger

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<http://bugs.python.org/issue7532>
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