inherit without calling parent class constructor?

[Christian Dieterich]

Hi,
I need to create many instances of a class D that inherits from a class 
B. Since the constructor of B is expensive I'd like to execute it only 
if it's really unavoidable. Below is an example and two workarounds, 
but I feel they are not really good solutions. Does somebody have any 
ideas how to inherit the data attributes and the methods of a class 
without calling it's constructor over and over again?

Thank,
Christian
Here's the "proper" example:
class B:
    def __init__(self, length):
        size = self.method(length)
        self.size = size
    def __str__(self):
        return 'object size = ' + str(self.size)
    def method(self, length):
        print 'some expensive calculation'
        return length
class D(B):
    def __init__(self, length):
        B.__init__(self, length)
        self.value = 1
if __name__ == "__main__":
    obj = D(7)
    obj = D(7)
Here's a workaround:
class B:
    def __init__(self, length):
        size = self.method(length)
        self.size = size
    def __str__(self):
        return 'object size = ' + str(self.size)
    def method(self, length):
        print 'some expensive calculation'
        return length
class D(B):
    def __init__(self, object):
        for key, value in object.__dict__.iteritems():
            setattr(self, key, value)
        self.value = 1
if __name__ == "__main__":
    tmp = B(7)
    obj = D(tmp)
    obj = D(tmp)
Here's another workaround:
Bsize = 0
class B:
    def __init__(self, length):
        size = self.method(length)
        self.size = size
        global Bsize
        Bsize = self.size
    def __str__(self):
        return 'object size = ' + str(self.size)
    def method(self, length):
        print 'some expensive calculation'
        return length
class D(B):
    def __init__(self, length):
        self.size = Bsize
        self.value = 1
if __name__ == "__main__":
    B(7)
    obj = D(9)
    obj = D(9)
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