code: def _split_css(media, domain):
      
      # Read internal property without creating new Media instance.
      if not media._css:
          return ImmutableMedia.empty_instance
  
      needs_local = domain == 'local'
      new_css = {}
      for medium, url in six.iteritems(media._css):
          if needs_local == _is_local(url):
              new_css.setdefault(medium, []).append(url)
  
      if not new_css:
          return ImmutableMedia.empty_instance
      else:
          return Media(css=new_css)
docs: Extract the local or external URLs from a Media object.