code: def test_context_manager(self):
      with dbm.ndbm.open(self.filename, "c") as db:
          db["ndbm context manager"] = "context manager"
  
      with dbm.ndbm.open(self.filename, "r") as db:
          self.assertEqual(list(db.keys()), [b"ndbm context manager"])
  
      with self.assertRaises(dbm.ndbm.error) as cm:
          db.keys()
      self.assertEqual(
          str(cm.exception), "DBM object has already been closed"
      )
  
imports: from test.support import import_helper
  from test.support import os_helper
  import os
  import unittest
  import dbm.ndbm
  from dbm.ndbm import error
label_desc: Use image node_id 6 for calling the DbmTestCase obj's underlying member method code with example usage: obj.test_context_manager() without return types