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Sat Mar 3 18:58:59 CET 2012 · https://mail.python.org/pipermail/python-dev/2012-March/117288.html

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.

-- 
--Guido van Rossum (python.org/~guido)

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[Python-Dev] slice subscripts for sequences and mappings