A list alternative to the very useful collections.defaultdict

defaultlist.py

class DefaultList(list):
    def __init__(self, factory=None):
        super(DefaultList, self).__init__()
        # Check if argument is callable
        if factory is not None:
            try:
                factory()
            except TypeError as e:
                raise e
        self._factory = factory

    def __repr__(self):
        return "%s(%s, %s)" % (self.__class__.__name__, self._factory, super(DefaultList, self).__repr__())

    def _fill(self, index):
        while len(self) <= index:
            if self._factory is None:
                self.append(self._factory)
            else:
                self.append(self._factory())

    def __setitem__(self, index, value):
        self._fill(index)
        super(DefaultList, self).__setitem__(index, value)

    def __getitem__(self, index):
        self._fill(index)
        return super(DefaultList, self).__getitem__(index)