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

Eli Bendersky eliben at gmail.com
Sun Mar 4 04:37:37 CET 2012

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 tpas* to include everything a 'unified' tpaseverything struct would need, or add a new tpaseverything slot in the type struct. Then add a tpflag 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 tpassequence and tpasmapping. (Let's not forget about tpassequence.sqconcat, tpasnumber.nbadd, tpassequence.sqrepeat and tpasnumber.nbmul either.)

There's nothing to unify, really, since PyMappingMethods is just a subset of PySequenceMethods:

typedef struct { lenfunc mp_length; binaryfunc mp_subscript; objobjargproc mp_ass_subscript; } PyMappingMethods;

with the small difference that in PySequenceMethods sq_item and sq_ass_item just accept numeric indices. However, if PySequenceMethods has the was_sq_sclies and was_sq_ass_slice fields are reinstated to accept a generic PyObject, PyMappingMethods will be a true subset.

If we look at the code, this becomes even clearer: in a full grep on the Python 3.3 source, there is no object that defines tp_as_mapping but does not also define tp_as_sequence, except Modules/_sqlite/row.c [I'm not familiar enough with the _sqlite module, but there's a chance it would make sense for the Row to be a sequence too].

Eli

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