code: def sprite_url(map, cache_buster=True):
      
      map = map.render()
      sprite_maps = _get_cache('sprite_maps')
      sprite_map = sprite_maps.get(map)
      if not sprite_map:
          log.error("No sprite map found: %s", map, extra={'stack': True})
      if sprite_map:
          url = '%s%s' % (config.ASSETS_URL, sprite_map['*f*'])
          if cache_buster:
              url += '?_=%s' % sprite_map['*t*']
          url = "url(%s)" % escape(url)
          return String.unquoted(url)
      return String.unquoted('')
docs: Returns a url to the sprite image.