Speed of attribute lookup (was Re: advice on programming style: is multiple inheritance bad?)

[Aahz]

In article <101rd69b9127s51 at news.supernews.com>,
John Roth <newsgroups at jhrothjr.com> wrote:
>"Joe Mason" <joe at notcharles.ca> wrote in message
>news:slrnc1r2id.ea6.joe at gate.notcharles.ca...
>> 
>> When profiling a large app at work, we discovered that in C++ accessing
>> every parent class after the first is quite slow.  We were able to just
>> flip the order of inheritance for some classes, but if you're planning a
>> complex use of multiple inheritance you should keep that in mind.  (I'm
>> just assuming it has the same effect in Python.)
>
>Actually, it doesn't. For new style classes, Python precomputes the
>method resolution order, so looking up a method is simply flipping
>through a list of dictionaries until it finds the right one. It's
>basically O(n) in the number of classes in the hierarchy.
>
>Given that you can add or remove attributes from any class at any time,
>it's hard to see how it could be any faster.

Actually, it's easy to see how it could be faster:

The problem with attribute lookup in new-style classes is that any
instance attribute could be a property.  So if Python finds an attribute
on the instance, it needs to traverse the MRO to check if there's a
property.  If there isn't one, that's an expensive waste.

Guido, Ping, and I worked on a patch to fix that at last year's PyCon
sprint, but one common case slowed down enough that Guido vetoed the
patch.  Anyone who wants to bring fresh eyeballs would be welcome.
(Don't remember the precise specifics off-hand, but I'm willing to work
with someone who wants to try.)
-- 
Aahz (aahz at pythoncraft.com)           <*>         http://www.pythoncraft.com/

"The joy of coding Python should be in seeing short, concise, readable
classes that express a lot of action in a small amount of clear code -- 
not in reams of trivial code that bores the reader to death."  --GvR