Python: multiple super classes only triggered 1 __init__ function, why?

Question

I've got a simple test program like this:

__metaclass__=type
class b1:
    def __init__(s):
        print "world"
class b2:
    def __init__(s):
        print "hello"
class d(b1,b2):
    def __init__(s):
        super(d,s).__init__()
obj=d()

I expect that "d" construting will print out both "b1" and "b2" base classes "init" content, and thus I'll see "world hello"

But actually the result was just "world". I'm using python2.4.5

Any error in my understanding?