def select_ships(ais, mmsi_list):
    """
    Filter out ship from ais which do not belongs to mmsi_list.

    Parameters
    ----------
    ais : pandas.DataFrame
        AIS data.
    mmsi_list : list
        List of wanted ships' MMSI.

    Returns
    -------
    pandas.DataFrame
        AIS data containing ship in mmsi_list.

    """
    mask = ais['mmsi'].isin(mmsi_list)
    ais = ais[mask]
    return ais
