[Python-Dev] super_getattro() Behaviour
mwh at python.net
Wed Apr 13 13:52:32 CEST 2005
"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.
> 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
> 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.
> 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
(well, ish, it might make more sense to only do that if the dict
For now, not being lazy seems your only option :-/ (it's what PyObjC
Many of the posts you see on Usenet are actually from moths. You
can tell which posters they are by their attraction to the flames.
-- Internet Oracularity #1279-06
More information about the Python-Dev