Re: inherit without calling parent class constructor?

[Steven Bethard]

Christian Dieterich wrote:
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)
I'm confused as to how you can tell when it's avoidable...  Do you mean 
you don't want to call 'method' if you don't have to?  Could you make 
size a property, e.g.

class B(object):
    def __init__(self, length):
        self._length = length
    def _get_size(self):
        print 'some expensive calculation'
        return self._length
    size = property(fget=_get_size)
class D(B):
    def __init__(self, length):
        super(B, self).__init__(length)
        self.value = 1
if __name__ == "__main__":
    obj = D(7)
    obj = D(7)
Then 'size' won't be calculated until you actually use it.  If 'size' is 
only to be calculated once, you might also look at Scott David Daniels's 
lazy property recipe:

http://aspn.activestate.com/ASPN/Cookbook/Python/Recipe/363602
Steve
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