[Python-Dev] super_getattro() Behaviour

[Michael Hudson]

"Phil Thompson" <phil at riverbankcomputing.co.uk> writes:

>>> Questions...
>>>
>>> 1. What is the reason why it doesn't go via tp_getattro?
>>
>> Because it wouldn't work if it did?  I'm not sure what you're
>> suggesting here.
>
> I'm asking for an explanation for the current implementation. Why wouldn't
> it work if it got the attribute via tp_getattro?

Well, using type->tp_getattro is just different to looking in tp_dict
-- it finds metamethods, for example.

Hmm.  Well, I'm fairly sure there is a difference, I'm not sure I can
explain it right now :(

>>> 2. A possible workaround is to subvert the ma_lookup function of the
>>> type
>>> dictionary after creating the type to do something similar to what my
>>> tp_getattro function is doing.

[...]

> Yes, having looked in more detail at the dict implementation I really
> don't want to go there.

Good :)

>>> 4. Am I missing a more correct/obvious technique? (There is no need to
>>> support classic classes.)
>>
>> Hum, I can't think of one, I'm afraid.
>>
>> There has been some vague talk of having a tp_lookup slot in
>> typeobjects, so
>>
>> PyDict_GetItem(t->tp_dict, x);
>>
>> would become
>>
>> t->tp_lookup(x);
>>
>> (well, ish, it might make more sense to only do that if the dict
>> lookup fails).
>
> That would be perfect. I can't Google any reference to a discussion - can
> you point me at something?

Well, most of the discussion so far has been in my head :)

There was a little talk of it in the thread "can we stop pretending
_PyType_Lookup is internal" here and possibly on pyobjc-dev around the
same time.

I'm not that likely to work on it soon -- I have enough moderately
complex patches to core Python I'm persuading people to think about
:-/.

>> For now, not being lazy seems your only option :-/ (it's what PyObjC
>> does).
>
> Not practical I'm afraid. I think I can only document that super doesn't
> work in this context.

Oh well.  I can't even think of a way to make it fail reliably...

Cheers,
mwh

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