def flatten_list(tier_list):
    """
    Given a list of lists, this returns a flat list of all items.

    :params list tier_list: A 2D list.

    :returns: A flat list of all items.
    """
    if tier_list is None:
        return []
    already_flattened = False
    for item in tier_list:
        if type(item) != list:
            already_flattened = True
    if already_flattened == True:
        return tier_list

    flat_list = [item for sublist in tier_list for item in sublist]
    return flat_list