old_contents: import collections
  from flask.sessions import SessionInterface, SessionMixin
  
  
  class NullSession(collections.MutableMapping, SessionMixin):
      """
      A session similar to the Flask's :class:`flask.sessions.NullSession`, but
      with a different error message.
      """
      def __len__(self):
          return 0
  
      def __getitem__(self, key):
          raise KeyError(key)
  
      def __iter__(self):
          return iter(())
  
      @staticmethod
      def _fail(*args, **kwargs):
          raise RuntimeError("Sessions are not supported.")
  
      __delitem__ = __setitem__ = _fail
  
  
  class NullSessionInterface(SessionInterface):
      def open_session(self, app, request):
          return NullSession()
  
      def save_session(self, app, session, response):
          pass
  
new_contents: import collections
  from flask.sessions import SessionInterface, SessionMixin
  
  
  class NullSession(collections.MutableMapping, SessionMixin):
      """
      A session similar to the Flask's :class:`flask.sessions.NullSession`, but
      with a different error message.
      """
      __slots__ = ()
  
      def __len__(self):
          return 0
  
      def __getitem__(self, key):
          raise KeyError(key)
  
      def __iter__(self):
          return iter(())
  
      @staticmethod
      def _fail(*args, **kwargs):
          raise RuntimeError("Sessions are not supported.")
  
      __delitem__ = __setitem__ = _fail
  
  
  class NullSessionInterface(SessionInterface):
      def open_session(self, app, request):
          return NullSession()
  
      def save_session(self, app, session, response):
          pass
  
subject: Add __slots__ to NullSession