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

Eli Bendersky eliben at gmail.com
Sat Mar 3 19🔞11 CET 2012

On Sat, Mar 3, 2012 at 19:58, Guido van Rossum <guido at python.org> wrote:

On Sat, Mar 3, 2012 at 4:20 AM, Antoine Pitrou <solipsis at pitrou.net> wrote:

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. That's long ago... IIRC it was for binary compatibility -- I didn't want to add an extra slot to the sq struct because it would require recompilation of 3rd party extensions. At the time that was an important concern.

Perhaps the situation can be fixed now without binary compatibility concerns. PySequenceMethods is:

typedef struct { lenfunc sq_length; binaryfunc sq_concat; ssizeargfunc sq_repeat; ssizeargfunc sq_item; void *was_sq_slice; ssizeobjargproc sq_ass_item; void *was_sq_ass_slice; objobjproc sq_contains;

binaryfunc sq_inplace_concat;
ssizeargfunc sq_inplace_repeat;

} PySequenceMethods;

The slots "was_sq_slice" and "was_sq_ass_slice" aren't used any longer. These can be re-incarnated to accept a slice object, and sequence objects can be rewritten to use them instead of implementing the mapping protocol (is there any reason listobject implements the mapping protocol, other than to gain the ability to use slices for getitem?). Existing 3rd party extensions don't need to be recompiled or changed, however. They can be, if their authors are interested, of course.

Eli

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