code: def test_HandleMissingIndicator_NanNoNanInTrain_ExpectAsNanColumn(
      self,
  ):
      train = ["A", "B"]
      test = ["A", "B", np.nan]
  
      encoder = encoders.HelmertEncoder(
          handle_missing="indicator", handle_unknown="value"
      )
      encoder.fit(train)
      result = encoder.transform(test)
  
      expected = [[1, -1, -1], [1, 1, -1], [1, 0, 2]]
      self.assertEqual(result.to_numpy().tolist(), expected)
  
file_path: category_encoders/tests/test_helmert.py
imports: import pandas
  from unittest import TestCase
  import numpy
  import category_encoders
label_desc: Use image node_id 9 for calling the TestHelmertEncoder obj's underlying member method code with example usage: obj.test_HandleMissingIndicator_NanNoNanInTrain_ExpectAsNanColumn() without return types