code: def test_combines_empty_non_empty(self):
      vectors = np.array([[-1, 3, 6], [2, -5, 8], [4, 7, -9]])
      accumulator1 = variance_util.MeanCovAccumulator()
      accumulator2 = variance_util.MeanCovAccumulator()
      accumulator2.update(vectors)
      accumulator1.merge(accumulator2)
      expected_mean = list(np.mean(vectors, axis=0))
      expected_covariance = list(np.cov(vectors, rowvar=False).ravel())
      actual_mean = list(accumulator1.mean)
      actual_covariance = list(accumulator1.covariance.ravel())
  
      self.assertListEqual(expected_mean, actual_mean)
      self.assertListEqual(expected_covariance, actual_covariance)
  
imports: from absl.testing import absltest
  from absl.testing import parameterized
  import numpy
  from tensorflow_data_validation.utils import variance_util
label_desc: Use image node_id 5 for calling the MeanCovAccumulatorTest obj's underlying member method code with example usage: obj.test_combines_empty_non_empty() without return types