code: def ping_pong(self, s1, s2, msg):
      s1.send(msg)
      msg2 = s2.recv()
      s2.send(msg2)
      msg3 = s1.recv()
      return msg3
  
imports: import sys
  import time
  from threading import Thread
  from unittest import TestCase
  from pytest import mark
  import zmq
  from zmq.utils import jsonapi
label_desc: Use image node_id 6 for calling the BaseZMQTestCase obj's underlying member method code with example usage: obj.ping_pong(s1, s2, msg) and returns: msg3