code: def open(self, options, loaded_schemata):
          
          if self.opened:
              return
          self.opened = True
          log.debug("%s, importing ns='%s', location='%s'", self.id, self.ns[1],
              self.location)
          result = self.__locate()
          if result is None:
              if self.location is None:
                  log.debug("imported schema (%s) not-found", self.ns[1])
              else:
                  url = self.location
                  if "://" not in url:
                      url = urljoin(self.schema.baseurl, url)
                  result = (loaded_schemata.get(url) or
                      self.__download(url, loaded_schemata, options))
          log.debug("imported:\n%s", result)
          return result
docs: Open and import the referenced schema.
  
          @param options: An options dictionary.
          @type options: L{options.Options}
          @param loaded_schemata: Already loaded schemata cache (URL --> Schema).
          @type loaded_schemata: dict
          @return: The referenced schema.
          @rtype: L{Schema}