Calling parent class __init__ with multiple inheritance, what’s the right way?

Say I have a multiple inheritance scenario:

class A(object):
    # code for A here

class B(object):
    # code for B here

class C(A, B):
    def __init__(self):
        # What's the right code to write here to ensure 
        # A.__init__ and B.__init__ get called?

There’s two typical approaches to writing C‘s __init__:

1. (old-style) ParentClass.__init__(self)
2. (newer-style) super(DerivedClass, self).__init__()

However, in either case, if the parent classes (A and B) don’t follow the same convention, then the code will not work correctly (some may be missed, or get called multiple times).

So what’s the correct way again? It’s easy to say “just be consistent, follow one or the other”, but if A or B are from a 3rd party library, what then? Is there an approach that can ensure that all parent class constructors get called (and in the correct order, and only once)?

Edit: to see what I mean, if I do:

class A(object):
    def __init__(self):
        print("Entering A")
        super(A, self).__init__()
        print("Leaving A")

class B(object):
    def __init__(self):
        print("Entering B")
        super(B, self).__init__()
        print("Leaving B")

class C(A, B):
    def __init__(self):
        print("Entering C")
        A.__init__(self)
        B.__init__(self)
        print("Leaving C")

Then I get:

Entering C
Entering A
Entering B
Leaving B
Leaving A
Entering B
Leaving B
Leaving C

Note that B‘s init gets called twice. If I do:

class A(object):
    def __init__(self):
        print("Entering A")
        print("Leaving A")

class B(object):
    def __init__(self):
        print("Entering B")
        super(B, self).__init__()
        print("Leaving B")

class C(A, B):
    def __init__(self):
        print("Entering C")
        super(C, self).__init__()
        print("Leaving C")

Then I get:

Entering C
Entering A
Leaving A
Leaving C

Note that B‘s init never gets called. So it seems that unless I know/control the init’s of the classes I inherit from (A and B) I cannot make a safe choice for the class I’m writing (C).