code: def test_list_studies(self):
      mock_list_studies = mock.MagicMock()
      expected_studies = {
          "studies": [
              {
                  "name": "1",
                  "study_config": self._study_config,
                  "state": "COMPLETED",
                  "create_time": {"seconds": 1234, "nanos": 0},
                  "inactive_reason": None,
              },
              {
                  "name": "2",
                  "study_config": self._study_config,
                  "state": "COMPLETED",
                  "create_time": {"seconds": 1235, "nanos": 0},
                  "inactive_reason": None,
              },
          ]
      }
      mock_list_studies.return_value.execute.return_value = (
          expected_studies
      )
      self._mock_discovery.projects().locations().studies().list = (
          mock_list_studies
      )
  
      studies = self._client.list_studies()
  
      mock_list_studies.assert_called_once_with(parent=self._parent)
      self.assertLen(studies, 2)
  
imports: from googleapiclient import errors
  import httplib2
  import mock
  import tensorflow
  from tensorflow_cloud.tuner import vizier_client
label_desc: Use image node_id 16 for calling the VizierClientTest obj's underlying member method code with example usage: obj.test_list_studies() without return types