DefaultList

#!/usr/bin/env python3
from collections import Callable

class DefaultList(list):
    def __init__(self, items=None, default=None, invoke_default=None):
        super().__init__(items or list())
        if (invoke_default
           or (invoke_default is None and isinstance(default, Callable))):
            self._default = default
        else:
            self._default = lambda: default

    def __getitem__(self, index):
        return (self._default() if index >= len(self)
                else super().__getitem__(index))

    def __setitem__(self, index, obj):
        if index >= len(self):
            n = index - len(self) + 1
            self.extend(self._default() for _ in range(n))
        super().__setitem__(index, obj)