At 08:23 PM 7/8/2007 +0300, Paul Pogonyshev wrote:
I would like to propose an optimization (I think so, anyway) for the way attributes are looked up. Currently, it is done like this:
return value of attribute in instance.__dict__ if present for type in instance.__class__.__mro__: return value of attribute in type.__dict__ if present raise AttributeError
Actually, it is only done like that for "classic" classes. New-style classes actually work more like this: descriptor = None for t in type(ob).__mro__: if attribute in t.__dict__: descriptor = t.__dict__[attribute] if hasattr(descriptor, '__set__'): return descriptor.__get__(ob, type(ob)) break if attribute in ob.__dict__: return ob.__dict__[attribute] if descriptor is not None: return descriptor.__get__(ob, type(ob)) if hasattr(type(ob),'__getattr__'): return ob.__getattr__(attribute) raise AttributeError
I propose adding to each type a C-implementation-private dictionary of attribute-name => type-in-which-defined. Then, it will not be necessary to traverse __mro__ on each attribute lookup for names which are present in this lookup dictionary.
Sounds good to me... but it's just as simple to store the descriptors directly, rather than the type that defines the descriptor. Might as well cut out the middleman. I believe that someone proposed this already, with a patch, in fact...
This optimization will not have any effect for attributes defined on instance.
It will for new-style classes, actually -- and a significant one if the inheritance hierarchy is deep and doesn't contain a default value for the attribute.
It will, however, for type attributes, most notably for methods.
Yep. It'll also speed up access to inherited slots.
It will most likely cause a slowdown for looking up attributes that are defined directly on self.__class__, not on any of its bases.
Not if it's a direct cache of descriptors; in that case it will have no effect on lookup time.
One open question is what to do in case an attribute on a type is set or deleted.
New-style classes can handle this easily; they know their subclasses and you can't directly write to a new-style class' __dict__. So when you set or delete an attribute on a type, it's possible to walk the subclasses and update their caches accordingly. I believe Python already does this so that if you e.g. set 'sometype.__call__ = something', then all the subclasses' C-level tp_call slots get changed to match. The same approach could be used for caching on new-style classes. Again, though, this has already been proposed, and I believe there's a patch awaiting review for inclusion in 2.6 (and presumably 3.0).