def initialize_grid(world_size, inner):
    """
    Creates an empty grid (2d list) with the dimensions specified in
    world_size. Each element is initialized to the inner argument.
    """
    data = []

    for i in range(world_size[1]):
        data.append([])
        for j in range(world_size[0]):
            data[i].append(deepcopy(inner))
    return data