To be clear, in this simple example I gave you don't have to override anything. However, if you want to process the values you place in the container in some way before turning on immutability (which I assume you must want to do because otherwise why not just use a tuple to begin with?), then that processing should take place in a.__new__.
cheers, Jess -- http://mail.python.org/mailman/listinfo/python-list
