code: def retrieve_taf(station_icao) -> typing.Tuple[typing.Union[str, None], typing.Union[str, None]]:
      
      url = _BASE_TAF_URL.format(station=station_icao)
      with requests.get(url) as resp:
          if not resp.ok:
              return f'unable to obtain TAF for station {station_icao}\n' \
                     f'Got to "http://tgftp.nws.noaa.gov/data/observations/metar/stations" ' \
                     f'for a list of valid stations', None
          return None, resp.content.decode().split('\n')[1]
docs: Retrieves a TAF string from an online database
  
      Args:
          station_icao: ICAO of the station
  
      Returns:
          tuple of error, metar_str