old_contents: class Shutter(object):
      def setup_database(self, pool):
          self.__dbpool = pool
      def snapshots(self, url):
          def _get_snapshot_urls(txn, url):
              txn.execute("""SELECT s.file_path
                               FROM shutter.urls u
                                   ,shutter.snapshots s
                              WHERE u.url = %s
                                AND u.id = s.url_id
                           ORDER BY s.created_at DESC
              """, [url])
              return txn.fetchall()
          return self.__dbpool.runInteraction(_get_snapshot_urls, url)
new_contents: class Shutter(object):
      def setup_database(self, pool):
          self.__dbpool = pool
      def snapshots(self, url):
          def _get_snapshot_urls(txn, url):
              txn.execute("""SELECT s.file_path
                               FROM shutter.urls u
                                   ,shutter.snapshots s
                              WHERE u.url = %s
                                AND u.id = s.url_id
                           ORDER BY s.created_at DESC
              """, [url])
              return [column[0] for column in txn.fetchall()]
          return self.__dbpool.runInteraction(_get_snapshot_urls, url)
subject: Remove additional nesting from Shutter.snapshots results