code: def test_encode_string_unknown_char():
      with pytest.raises(ValueError):
          utils.encode_string("abc", "xyz")
  
file_path: doctr/tests/common/test_datasets_utils.py
imports: import numpy
  import pytest
  from doctr.datasets import utils
label_desc: Use image node_id 4 for calling a global function with example usage: test_encode_string_unknown_char() without return types