code: def test_master_sdown(cluster, sentinel):
      cluster.master["is_sdown"] = True
      with pytest.raises(MasterNotFoundError):
          sentinel.discover_master("mymaster")
  
imports: import socket
  from unittest import mock
  import pytest
  import redis.sentinel
  from redis import exceptions
  from redis.sentinel import MasterNotFoundError, Sentinel, SentinelConnectionPool, SlaveNotFoundError
label_desc: Use image node_id 11 for calling a global function with example usage: test_master_sdown(cluster, sentinel) without return types