def get_nearest_site(sites, lat, lon):
    """
    Return a string which can be used as "request" in calls to loc_forecast
    and loc_observations.
    
    sites:    List of Site instances
    lat:      float or int.  Interesting latitude
    lon:      float or int.  Interesting longitude
    """
    for site in sites:
        site.distance_to_coords(lat, lon)
    sites.sort(key=operator.attrgetter("distance"))
    return sites[0].ident