Is it possible to have instance variables in subclasses of builtins?

[Kenneth McDonald]

I've recently used subclasses of the builtin str class to good effect.
However, I've been unable to do the following:

1) Call the constructor with a number of arguments other than
the number of arguments taken by the 'str' constructor.

2) Create and use instance variables (eg. 'self.x=1') in
the same way that I can in a 'normal' class.

As an example of what I mean, here's a short little session:

>>> class a(str):
...     def __init__(self, a, b):
...             str.__init__(a)
...             self.b = b
... 
>>> x = a("h", "g")
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
  TypeError: str() takes at most 1 argument (2 given)
>>> 
  
(hmm, on reflection, I shouldn't have used 'a' as a
parameter to __init__--but that's merely bad
style, not the cause of the error :-) )

On the other hand, this example works:

>>> class b(str):
...     def __init__(self, x):
...             str.__init__(x)
... 
>>> x = b("h")
>>> x
'h'
>>> 

Is it possible to circumvent the above restrictions?

Thanks,
Ken