Class X(object):
    def notify(self):
        print "notifying..."

Class A(object):
    def __init__(self):
        self.x = X()
        self.b = B(self.x)
        self.d = D(self.x)

Class B(object):
    def __init__(self, x):
        self.x = x

    def notify(self):
        self.x.notify('from B')

Class D(object):
    def __init__(self, x):
        self.x = x

    def notify(self):
        self.x.notify('from D')

a = A()