defaultdict equivalent for lists

Question

Is there\How would you build an equivalent of python's very useful collections.defaultdict?

Imagined usage of such a container:

>>> a = collections.defaultlist(0)
>>> a[2]=7
>>> a[4]='x'
>>> a
[0,0,7,0,'x']

UPDATE: I've added a follow up question to add even more functionality to this construct

Answer 1

I think this would be a bit confusing to use; however, here's my first thought on how to do it:

class defaultlist(list):
    def __init__(self, fx):
        self._fx = fx

    def __setitem__(self, index, value):
        while len(self) <= index:
            self.append(self._fx())
        list.__setitem__(self, index, value)

This takes a callable (I think that's how defaultdict works) for the default value.

When I run:

a = defaultlist(int)
print a
a[2] = 7
a[4] = 'x'
print a

I get back:

[]
[0, 0, 7, 0, 'x']