[Python-Dev] slice subscripts for sequences and mappings (original) (raw)

Eli Bendersky eliben at gmail.com
Sat Mar 3 13:41:24 CET 2012

I'd expect slice subscripts to be part of the sequence interface, and yet they are not. In fact, they are part of the mapping interface. For example, the list object has its slice get/set methods assigned to a PyMappingMethods struct. So does a bytes object, and pretty much every other object that wants to support subscripts. It comes from: http://hg.python.org/cpython/rev/245224d1b8c9 http://bugs.python.org/issue400998 Written by Michael Hudson and reviewed by Guido. I wonder why this patch chose to add mapping protocol support to tuples and lists, rather than add a tp slot for extended slicing.

Why a separate tp_ slot for extended slicing? ISTM slicing pertains to sequences, similarly to other numeric indices. If you look at PySequenceMethods it has these (apparently no longer used fields):

void *was_sq_slice;
void *was_sq_ass_slice;

These were "simple" slices (pairs of numbers). I suppose if any change is considered, these fields can be re-incarnated to accept PyObject* slices similarly to the current mp_subscript and mp_ass_subscript.

Eli

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