Unpythonic? Impossible??

[BrJohan]

Assume having this class hierarchy: (in principle and without details)
    class A(object):
    class B1(A):
    class B2(A):
    class C1(A1):
    class C2(A1):
    class C3(B1):
    class C4(B2):

each of those classes have an initializer __init__(self, data):  and an 
overloaded __new__(cls, data):

is it then possible to have this call:
    obj = A(data)
return an instance of that particular class (e.g. class C3)  in the 
hierarchy that - as decided by the __new__ functions - is the 'correct' one?

A.__new__ could select between A, B1 and B2, while B1.__new__ could choose 
from B1, C3 and C4.

I know how to use a class factory - and could work around using such a 
mechanism. However I am interested to know if I could let the classes do the 
work by themselves.

/BJ