Guido van Rossum wrote: >On 10/1/05, Travis Oliphant <[EMAIL PROTECTED]> wrote: > > >>The new ndarray object of scipy core (successor to Numeric Python) is a >>C extension type that has a getitem defined in both the as_mapping and >>the as_sequence structure. >> >>The as_sequence mapping is just so PySequence_GetItem will work correctly. >> >>As exposed to Python the ndarray object has a .__getitem__ wrapper method. >> >>Why does this wrapper call the sequence getitem instead of the mapping >>getitem method? >> >>Is there anyway to get at a mapping-style __getitem__ method from Python? >> >> > >Hmm... I'm sure the answer is in typeobject.c, but that is one of the >more obfuscated parts of Python's guts. I wrote it four years ago and >since then I've apparently lost enough brain cells (or migrated them >from language implementation to to language design service :) that I >don't understand it inside out any more like I did while I was in the >midst of it. > >However, I wonder if the logic isn't such that if you define both >sq_item and mp_subscript, __getitem__ calls sq_item; I wonder if by >removing sq_item it might call mp_subscript? Worth a try, anyway. > > >
Thanks for the tip. I think I figured out the problem, and it was my misunderstanding of how types inherit in C that was the source of my problem. Basically, Python is doing what you would expect, the mp_item is used for __getitem__ if both mp_item and sq_item are present. However, the addition of these descriptors (and therefore the resolution of any comptetion for __getitem__ calls) is done *before* the inheritance of any slots takes place. The new ndarray object inherits from a "big" array object that doesn't define the sequence and buffer protocols (which have the size limiting int dependencing in their interfaces). The ndarray object has standard tp_as_sequence and tp_as_buffer slots filled. Figuring the array object would inherit its tp_as_mapping protocol from "big" array (which it does just fine), I did not explicitly set that slot in its Type object. Thus, when PyType_Ready was called on the ndarray object, the tp_as_mapping was NULL and so __getitem__ mapped to the sequence-defined version. Later the tp_as_mapping slots were inherited but too late for __getitem__ to be what I expected. The easy fix was to initialize the tp_as_mapping slot before calling PyType_Ready. Hopefully, somebody else searching in the future for an answer to their problem will find this discussion useful. Thanks for your help, -Travis _______________________________________________ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
