code: def google_app_engine_ndb_delete_expired_sessions(dormant_for=86400, limit=500):
      
      from vishnu.backend.client.google_app_engine_ndb import VishnuSession
      from google.appengine.ext import ndb
      from datetime import datetime
      from datetime import timedelta
  
      now = datetime.utcnow()
      last_accessed = now - timedelta(seconds=dormant_for)
  
      query = VishnuSession.query(ndb.OR(
          ndb.AND(VishnuSession.expires <= now, VishnuSession.expires != None),
          VishnuSession.last_accessed <= last_accessed
      ))
      results = query.fetch(keys_only=True, limit=limit)
  
      ndb.delete_multi(results)
  
      return len(results) < limit
docs: Deletes expired sessions
      A session is expired if it expires date is set and has passed or
      if it has not been accessed for a given period of time.
  
      :param dormant_for: seconds since last access to delete sessions, defaults to 24 hours.
      :type dormant_for: int
      :param limit: amount to delete in one call of the method, the maximum and default for this is the NDB fetch limit of 500
      :type limit: int