code: def test_HandleUnknown_HaveOnlyKnown_ExpectSecondColumn(self):
      train = ["A", "B"]
  
      encoder = encoders.BackwardDifferenceEncoder(
          handle_unknown="indicator"
      )
      result = encoder.fit_transform(train)
  
      expected = [[1, -2 / 3.0, -1 / 3.0], [1, 1 / 3.0, -1 / 3.0]]
      self.assertEqual(result.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 11 for calling the TestBackwardsEncoder obj's underlying member method code with example usage: obj.test_HandleUnknown_HaveOnlyKnown_ExpectSecondColumn() without return types