code: def testExtractCharTokens(self, word_counts, expected):
      actual = learner.extract_char_tokens(word_counts)
      self.assertEqual(expected, actual)
  
imports: import collections
  import logging
  from absl.testing import absltest
  from absl.testing import parameterized
  import numpy
  from tensorflow.python.data.ops import dataset_ops
  from tensorflow.python.ops.ragged import ragged_string_ops
  from tensorflow_text.tools.wordpiece_vocab import wordpiece_tokenizer_learner_lib
label_desc: Use image node_id 1 for calling the ExtractCharTokensTest obj's underlying member method code with example usage: obj.testExtractCharTokens(word_counts, expected) without return types