Default list in python, á la defaultdict.

default_list.py

class DefaultList(list):
    """
    List that always returns a value

    If you try to access an missing index, it will fill up this list with
    empty lists for all missing indices before returning the requested
    index. This will be the last element since lists are ordered.

    A consequence of this is that `for` loops over this list will never
    stop.
    """

    def __getitem__(self, index):
        try:
            # This also handles slice objects, which always will return a
            # new list, and can thus never fail
            return super().__getitem__(index)
        except IndexError:
            # Fill the list with empty ones upto the requested index
            self[len(self):index + 1] = ([] for _ in range(len(self), index + 1))
            return super().__getitem__(index)