+1 on changing __dynamic__ or at least enabling some kind of class variable mutability by default.
After converting the Tools/compiler package to the new class system, I tend to agree (I already said so on c.l.py in my last post there).
Here's a problem, related to performance.
The dispatch for special operations (like __iter__) from C uses a C function stored in a corresponding slot (e.g. tp_iter) in the type object. So when class X has an __iter__ method, there needs to be a slot wrapper (a C function that calls the __iter__ object) in the in the tp_iter slot in the type object representing X.
For dynamic classes, I must assume that at any time someone can add a special method (like __iter__) to a class. It's hard to set up things so that the dispatch function is set in the type object at the moment C.__iter__ is assigned: it would require a class to keep track of all its subclasses, without keeping those subclasses alive. While I know I can do that using weak references, I don't like having to maintain all that administration. So at the moment, when a class is dynamic, I just stick all dispatch functions in the type object -- the dispatch functions will raise AttributeError when their corresponding method is not found. (This is the same approach used for classic classes, BTW.)
This is slower than it should be -- the fully dynamic Tools/compiler package compiles itself about 25% slower this way. If I tweak it to use all static classes (not very hard), it runs at about the same speed as it does with classic classes. I imagine I could make it faster by using __slots__, but I don't know enough about the internals yet to be able to do that.
My goal (before I'm happy with making __dynamic__=1 the default) is that dynamic classes should be at least as fast as classic classes. I haven't profiled it yet -- it's possible that there's a cheap hack possible by making more conservative assumptions about __getattr__ alone -- classic classes special-case __getattr__ too.)
--Guido van Rossum (home page: http://www.python.org/%7Eguido/)