code: def test_backwards_difference_encoder_preserve_dimension_3(self):
      train = ["A", "B", "C"]
      test = ["A", "B", "C", None]
  
      encoder = encoders.BackwardDifferenceEncoder(
          handle_unknown="value", handle_missing="value"
      )
      encoder.fit(train)
      test_t = encoder.transform(test)
  
      expected = [
          [1, -2 / 3.0, -1 / 3.0],
          [1, 1 / 3.0, -1 / 3.0],
          [1, 1 / 3.0, 2 / 3.0],
          [1, 0, 0],
      ]
      self.assertEqual(test_t.to_numpy().tolist(), expected)
  
file_path: category_encoders/tests/test_backward_difference.py
imports: import pandas
  from unittest import TestCase
  import numpy
  import category_encoders
label_desc: Use image node_id 3 for calling the TestBackwardsEncoder obj's underlying member method code with example usage: obj.test_backwards_difference_encoder_preserve_dimension_3() without return types