[Python-Dev] super_getattro() Behaviour

[Michael Hudson]

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

> In PyQt, wrapped types implement lazy access to the type dictionary
> through tp_getattro. If the normal attribute lookup fails, then private
> tables are searched and the attribute (if found) is created on the fly and
> returned. It is also put into the type dictionary so that it is found next
> time through the normal lookup. This is done to speed up the import of,
> and the memory consumed by, the qt module which contains thousands of
> class methods.
>
> This all works fine - except when super is used.
>
> The implementation of super_getattro() doesn't use the normal attribute
> lookup (ie. doesn't go via tp_getattro). Instead it walks the MRO
> hierarchy itself and searches instance dictionaries explicitly. This means
> that attributes that have not yet been referenced (ie. not yet been cached
> in the type dictionary) will not be found.
>
> 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.

> 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.

Eek!

> Are there any inherent problems with that?

Well, I think the layout of dictionaries is fiercely private.  IIRC,
the only reason it's in a public header is to allow some optimzations
in ceval.c (though this isn't at all obvious from the headers, so
maybe I'm mistaken).

> 3. Why, when creating a new type and eventually calling type_new() is a
> copy of the dictionary passed in made?

I think this is to prevent changes to tp_dict behind the type's back.
It's important to keep the dict and the slots in sync.

> Why not take a reference to it?  This would allow a dict sub-class
> to be used as the type dictionary. I could then implement a
> lazy-dict sub-class with the behaviour I need.

Well, not really, because super_getattro uses PyDict_GetItem, which
doesn't respect subclasses...

> 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).

For now, not being lazy seems your only option :-/ (it's what PyObjC
does).

Cheers,
mwh

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