Class hierarchy problem

[BrJohan]

I'm in need of help to solve this Python (ver. 3.3) problem:

I have a hierarchy of classes (SubA, SubAB, SubB, ..., SubBCA, 
SubC,...), each of which is inheriting from a chain of superclasses with 
a common baseclass(Sup) on top. (So far, no problem)

Now, I want to create instances of the correct subclasstype as decided 
by the common baseclass, like this:
	
i = Sup(args_allowing_the_baseclass_to_deduce_correct_subclass)

where i can be of any class except Sup itself (as decided by Sup)

Now, the problem:

How to design the __new__() and __init__() methods for the various 
classes in order to achieve what I want?

(Some ten years I had the same problem (in a different context) and was 
helped by asking in this group. However, the solution has disappeared. 
Maybe the 2.x solution is not the same as in 3.x?)