[Python-Dev] Why does __getitem__ slot of builtin call sequence methods first?
mwh at python.net
Sun Oct 2 22:36:01 CEST 2005
Travis Oliphant <oliphant at ee.byu.edu> writes:
> 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
> 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.
Oof. That'd do it.
> 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.
I guess the reason this hasn't come up before is that non-trivial C
inheritance is still pretty rare.
> 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.
Well, it sounds like a bug that should be easy to fix. I can't think
of a reason to do slot wrapper generation before slot inheritance,
though I wouldn't like to bet more than a beer on not having missed
There are two kinds of large software systems: those that evolved
from small systems and those that don't work.
-- Seen on slashdot.org, then quoted by amk
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