[Python-Dev] super_getattro() Behaviour

Phil Thompson phil at riverbankcomputing.co.uk
Wed Apr 13 11:41:24 CEST 2005

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? Bug or feature?

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?

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

4. Am I missing a more correct/obvious technique? (There is no need to
support classic classes.)

Many thanks,

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