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

Antoine Pitrou solipsis at pitrou.net
Sat Mar 3 22:02:50 CET 2012

On Sat, 3 Mar 2012 12:59:13 -0800 Thomas Wouters <thomas at python.org> wrote:

Why even have separate tpassequence and tpasmapping 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 PyMappingCheck() and PySequenceCheck() 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 tpassequence (in order to support containtment tests) and that PySequenceCheck() 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.

Regards

Antoine.

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