human
Complete the below
def closest_station(lat, lon, cache_dir):

    ''''Return the ZONE_ID.WMO_ID of the closest station to our lat/lon.'
    '''

Give me a the Python code you requested?
assistant
#Here is the Python code you requested.
    if ((lat is None) or (lon is None) or (not os.path.isdir(cache_dir))):
        return
    stations = zamg_stations(cache_dir)
    def comparable_dist(zamg_id):
        'Calculate    the    psudeo-distance    from    lat/lon.'
        (station_lat, station_lon) = stations[zamg_id]
        return (((lat - station_lat) ** 2) + ((lon - station_lon) ** 2))
    return min(stations, key=comparable_dist)