return dictionary mapping mode numbers to name, or None if unknown

def mode_mapping_bynumber(mav_type):
    '''return dictionary mapping mode numbers to name, or None if unknown'''
    map = None
    if mav_type in [mavlink.MAV_TYPE_QUADROTOR,
                    mavlink.MAV_TYPE_HELICOPTER,
                    mavlink.MAV_TYPE_HEXAROTOR,
                    mavlink.MAV_TYPE_OCTOROTOR,
                    mavlink.MAV_TYPE_COAXIAL,
                    mavlink.MAV_TYPE_TRICOPTER]:
        map = mode_mapping_acm
    if mav_type == mavlink.MAV_TYPE_FIXED_WING:
        map = mode_mapping_apm
    if mav_type == mavlink.MAV_TYPE_GROUND_ROVER:
        map = mode_mapping_rover
    if mav_type == mavlink.MAV_TYPE_ANTENNA_TRACKER:
        map = mode_mapping_tracker
    if map is None:
        return None
    return map