def args_parser() -> argparse.ArgumentParser:
    """Anomalies in Tor Usage.

    This program implements the method described in the paper at
    https://arxiv.org/abs/1507.05819.

    It takes Tor usage data from
    https://metrics.torproject.org/userstats-relay-country.csv and outputs a
    ranking of countries according to the level of detected anomalous behaviour
    in a date interval, as well as a graph showing a greater date interval.

    """
    parser = argparse.ArgumentParser(
        formatter_class=argparse.ArgumentDefaultsHelpFormatter,
        description=__doc__,
    )
    parser.add_argument(
        "-p",
        "--path",
        type=valid_file_path,
        default=None,
        help="Path to a data csv file, instead of downloading one.",
    )
    parser.add_argument(
        "-s",
        "--start-date",
        type=valid_date,
        default=default_start_date(),
        help="Start date to find anomalies in YYYY-MM-DD format.",
    )
    parser.add_argument(
        "-e",
        "--end-date",
        type=valid_date,
        default=default_end_date(),
        help="End date to find anomalies in YYYY-MM-DD format.",
    )
    parser.add_argument(
        "-t",
        "--train-start-date",
        type=valid_date,
        default=default_train_start_date(),
        help="Train data start date in YYYY-MM-DD format.",
    )
    parser.add_argument(
        "-u",
        "--train-end-date",
        type=valid_date,
        default=default_end_date(),
        help="Train data end date in YYYY-MM-DD format.",
    )
    parser.add_argument(
        "-w",
        "--write",
        action="store_true",
        default=False,
        help="Write partial matrices to files.",
    )
    parser.add_argument(
        "-r",
        "--read",
        action="store_true",
        default=False,
        help="Read partial matrices from files (if they exists).",
    )
    return parser