code: def test_splits_into_characters(self):
      tokenizer = CharacterTokenizer(
          start_tokens=["<S1>", "<S2>"], end_tokens=["</S2>", "</S1>"]
      )
      sentence = "A, small sentence."
      tokens = [t.text for t in tokenizer.tokenize(sentence)]
      expected_tokens = [
          "<S1>",
          "<S2>",
          "A",
          ",",
          " ",
          "s",
          "m",
          "a",
          "l",
          "l",
          " ",
          "s",
          "e",
          "n",
          "t",
          "e",
          "n",
          "c",
          "e",
          ".",
          "</S2>",
          "</S1>",
      ]
      assert tokens == expected_tokens
  
file_path: allennlp/tests/data/tokenizers/character_tokenizer_test.py
imports: from allennlp.common import Params
  from allennlp.common.testing import AllenNlpTestCase
  from allennlp.data.tokenizers import CharacterTokenizer
label_desc: Use image node_id 1 for calling the TestCharacterTokenizer obj's underlying member method code with example usage: obj.test_splits_into_characters() without return types