groovy - scraping polygons from OpenStreetMap (fr) API

GeoUtils.groovy

import groovy.json.JsonSlurper

class GeoUtils {
    static def retrieveProbableOsmRelationId(String cityName) {
        def cityNameEncoded = java.net.URLEncoder.encode(cityName, "utf-8")

        def queryUrl = "https://nominatim.openstreetmap.org/search?q=${cityNameEncoded}&format=json&osm_type=relation"
        def nominatimResult = new JsonSlurper().parseText(new URL(queryUrl).text)
        if (nominatimResult.size() == 0) {
            return null
        }
        return nominatimResult[0].osm_id
    }

    static def downloadRelationAsGeoJSON(def relationId) {
        def geoJsonUrl = new URL("http://polygons.openstreetmap.fr/get_geojson.py?id=${relationId}&params=0")
        def geoJsonText = geoJsonUrl.text
        if (geoJsonText == "None\n")
            throw new Exception("polygons.openstreetmap.fr returned an empty response")
        geoJsonUrl.text
    }

    static def grabGeoJsonFromOpenStreetMap(String cityName) {
        def relationId = retrieveProbableOsmRelationId(cityName)
        if (relationId == null)
            throw new Exception("not found")

        downloadRelationAsGeoJSON(relationId)
    }

}

static void main(String[] args) {
   def city = args[0]
   def geojson = GeoUtils.grabGeoJsonFromOpenStreetMap(city)
   println geojson

}