gistfile1.py

class InfiniteList:
    def __init__(self, init_val=1, next_val=1):
        self.real_list = []
        self.init_val = init_val
        self.diff = next_val - init_val

    def __getitem__(self, index):
        if index < 0:
            raise IndexError('Out of index range.')

        if index < len(self.real_list):
            return self.real_list[index]

        length = len(self.real_list)
        if length == 0:
            self.real_list.append(self.init_val)
        while length <= index:
            self.real_list.append(self.real_list[-1] + self.diff)
            length += 1
        return self.real_list[index]

    def __str__(self):
        return str(self.real_list)