[Python-Dev] slice subscripts for sequences and mappings

[Thomas Wouters]

On Sat, Mar 3, 2012 at 13:02, Antoine Pitrou <solipsis at pitrou.net> wrote:

> On Sat, 3 Mar 2012 12:59:13 -0800
> Thomas Wouters <thomas at python.org> wrote:
> >
> > Why even have separate tp_as_sequence and tp_as_mapping anymore? That
> > particular distinction never existed for Python types, so why should it
> > exist for C types at all? I forget if there was ever a real point to it,
> > but all it seems to do now is create confusion, what with many sequence
> > types implementing both, and PyMapping_Check() and PySequence_Check()
> doing
> > seemingly random things to come up with somewhat sensible answers.
>
> Ironically, most of the confusion stems from sequence types
> implementing the mapping protocol for extended slicing.
>
> > Do note
> > that the dict type actually implements tp_as_sequence (in order to
> support
> > containtment tests) and that PySequence_Check() has to explicitly return
> 0
> > for dicts -- which means that it will give the "wrong" answer for another
> > type that behaves exactly like dicts.
>
> It seems to be a leftover:
>
> int
> PySequence_Check(PyObject *s)
> {
>    if (PyDict_Check(s))
>        return 0;
>    return s != NULL && s->ob_type->tp_as_sequence &&
>        s->ob_type->tp_as_sequence->sq_item != NULL;
> }
>
> Dict objects have a NULL sq_item so even removing the explicit check
> would still return the right answer.
>
> > Getting rid of the misleading distinction seems like a much better idea
> > than trying to re-conflate some of the issues.
>
> This proposal sounds rather backwards, given that we now have separate
> Mapping and Sequence ABCs.
>

I'm not sure how the ABCs, which are abstract declarations of semantics,
tie into this specific implementation detail. ABCs work just as well for
Python types as for C types, and Python types don't have this distinction.
The distinction in C types has been *practically* useless for years, so why
should it stay? What is the actual benefit here?

-- 
Thomas Wouters <thomas at python.org>

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