code: def test_single_observations(self):
      vectors1 = np.array([[1, 2, 3]])
      vectors2 = np.array([[4, 5, 6]])
      vectors3 = np.array([[7, 8, 9]])
      accumulator1 = variance_util.MeanCovAccumulator()
      accumulator2 = variance_util.MeanCovAccumulator()
      accumulator3 = variance_util.MeanCovAccumulator()
  
      accumulator1.update(vectors1)
      self.assertListEqual([1, 2, 3], list(accumulator1.mean))
      self.assertListEqual(
          [0, 0, 0, 0, 0, 0, 0, 0, 0],
          list(accumulator1.covariance.ravel()),
      )
  
      accumulator1.update(vectors2)
      self.assertListEqual([2.5, 3.5, 4.5], list(accumulator1.mean))
      self.assertListEqual(
          [4.5, 4.5, 4.5, 4.5, 4.5, 4.5, 4.5, 4.5, 4.5],
          list(accumulator1.covariance.ravel()),
      )
  
      accumulator3.update(vectors3)
      accumulator2.merge(accumulator3)
      self.assertListEqual([7, 8, 9], list(accumulator2.mean))
      self.assertListEqual(
          [0, 0, 0, 0, 0, 0, 0, 0, 0],
          list(accumulator2.covariance.ravel()),
      )
  
      accumulator1.merge(accumulator2)
      self.assertListEqual([4, 5, 6], list(accumulator1.mean))
      self.assertListEqual(
          [9, 9, 9, 9, 9, 9, 9, 9, 9],
          list(accumulator1.covariance.ravel()),
      )
  
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 4 for calling the MeanCovAccumulatorTest obj's underlying member method code with example usage: obj.test_single_observations() without return types