Calling a derived class's constructor from a parent method

[jason]

If I have a class hierarchy like so:

 
class A(object):
      def __init__(self, s):
            self.s = s
      def foo(self, s):
            return A(s)

class B(A):
      def __init__(self, s):
            A.__init__(self, s)

If I make a B:

b = B(0)

I'd like b.foo(1) to return an instance of B. Is there a way to do that besides implementing a construct(self, s) for each part of the hierarchy? I.e. is there a way for the base class to look at self and find out what type to create?

I'm using Python 2.7.5, but I'm curious what the 3.x answer is too.