code: def is_disconnected(self, node_id):
          
          conn = self._conns.get(node_id)
          if conn is None:
              return False
          return conn.disconnected()
docs: Check whether the node connection has been disconnected or failed.
  
          A disconnected node has either been closed or has failed. Connection
          failures are usually transient and can be resumed in the next ready()
          call, but there are cases where transient failures need to be caught
          and re-acted upon.
  
          Arguments:
              node_id (int): the id of the node to check
  
          Returns:
              bool: True iff the node exists and is disconnected