code: def test_string_index(self):
      train = pd.DataFrame({"city": ["chicago", "denver"]})
      target = [0, 1]
      train.index = train.index.astype(str)
  
      for encoder_name in encoders.__all__:
          with self.subTest(encoder_name=encoder_name):
              enc = getattr(encoders, encoder_name)()
              result = enc.fit_transform(train, target)
              self.assertFalse(
                  result.isna().any(axis=None),
                  "There should not be any missing value!",
              )
  
imports: import warnings
  from datetime import timedelta
  import numpy
  from numpy.testing import assert_array_equal
  import pandas
  import sklearn
  import tests.helpers
  from sklearn.utils.estimator_checks import check_transformer_general, check_transformers_unfitted, check_n_features_in
  from sklearn.compose import ColumnTransformer
  from unittest import TestCase
  from copy import deepcopy
  import category_encoders
label_desc: Use image node_id 28 for calling the TestEncoders obj's underlying member method code with example usage: obj.test_string_index() without return types