code: def GetUsername(self, event, default_username='-'):
      
      username = getattr(event, 'username', None)
      if username and username != '-':
        return username
  
      session_identifier = event.GetSessionIdentifier()
      if session_identifier is None:
        return default_username
  
      user_sid = getattr(event, 'user_sid', None)
      username = self._knowledge_base.GetUsernameByIdentifier(
          user_sid, session_identifier=session_identifier)
      return username or default_username
docs: Retrieves the username related to the event.
  
      Args:
        event (EventObject): event.
        default_username (Optional[str]): default username.
  
      Returns:
        str: username.