A simple list implementation that yields None or a defined default if an out-of-bounds index is referenced.

DefaultList.py

class DefaultList(list):
    def __init__(self, iterable=[], default=None):
        super(self.__class__, self).__init__(iterable)
        
        self.default = default
        
    def __getitem__(self, index):
        """
        If 'index' is outside of the bounds (including negative indexing) of the list,
        return 'self.default.'
        
        :param: index (int): The numerical index to return.
        
        :return: (any) The element at index 'index.'
            If that index doesn't exist, 'self.default.'
        """
        
        length = len(self)
        
        if index in range(-1 * length, length):
            return super(self.__class__, self).__getitem__(index)
        else:
            return self.default