Diamond inheritance with str

[Patrick Earl]

Hi.  I have the need to inherit from multiple classes that share the 
same base class.  This base class is a built in type like str, list, 
float, or int.

The following works:

>>> class A(list): pass
...
>>> class B(list): pass
...
>>> class C(A,B): pass
...
>>>

It also seems to work for int and float.  However, the following does 
not work:

>>> class A(str): pass
...
>>> class B(str): pass
...
>>> class C(A,B): pass
...
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
TypeError: multiple bases have instance lay-out conflict

Why doesn't this inheritance structure work for strings?  Is this a 
bug?

Is there a good way to work around this?  Using UserString allows the 
inheritance, but I thought that UserString was considered obsolete due to 
the ability to subclass built-in types.

I appreciate any thoughts on the matter.  Thanks. :)

        Patrick

PS.  My apologies if this is a duplicate.  I'm pretty sure it's not, but I 
made a posting to python-list at python.org that didn't appear to go through.