creating a defaultlist in python

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On the example you give, you first try to retrieve a non-existing value on the list, as you do dl[2]['a'], Python first retrieve the third (index 2) element on the list, then proceed to get the element named ‘a’ on that object – therefore you have to implement your automatic extending behavior to the __getitem__ method as well, like this:

class defaultlist(list):
    def __init__(self, fx):
        self._fx = fx
    def _fill(self, index):
        while len(self) <= index:
            self.append(self._fx())
    def __setitem__(self, index, value):
        self._fill(index)
        list.__setitem__(self, index, value)
    def __getitem__(self, index):
        self._fill(index)
        return list.__getitem__(self, index)

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