def scenario_agencies(data_dir, scenario_name):
    """Given root data directory and scenario name, computes a mapping
    of agency names to their respective paths.

    Parameters
    ----------
    data_dir : Path
        Absolute path to root of data directory
    scenario_name : str
        Name of scenario with GTFS data

    Returns
    -------
    dict
        Dictionary of agency names mapped to directories containing files comprising their GTFS
        data.
    """
    gtfs_root = (data_dir / scenario_name).absolute()
    return {p.stem: p for p in
            gtfs_root.iterdir()}