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

Thomas Wouters thomas at python.org
Sun Mar 4 03:24:40 CET 2012

On Sat, Mar 3, 2012 at 13:12, Stefan Behnel <stefan_ml at behnel.de> wrote:

Thomas Wouters, 03.03.2012 21:59: > 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. 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. > > Getting rid of the misleading distinction seems like a much better idea > than trying to re-conflate some of the issues.

We're too far away from the release of Python 4 to change something with that kind of impact, though.

It's not hard to do this in a backward-compatible way. Either grow one of the tp_as_* to include everything a 'unified' tp_as_everything struct would need, or add a new tp_as_everything slot in the type struct. Then add a tp_flag to indicate that the type has this new layout/slot and guard all uses of the new slots with a check for that flag. If the type doesn't have the new layout or doesn't have it or the slots in it set, the code can fall back to the old try-one-and-then-the-other behaviour of dealing with tp_as_sequence and tp_as_mapping.

(Let's not forget about tp_as_sequence.sq_concat, tp_as_number.nb_add, tp_as_sequence.sq_repeat and tp_as_number.nb_mul either.)

-- Thomas Wouters <thomas at python.org>

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