code: def test_add_feature(feature_names):
      len_before = len(feature_names)
      storage = feature_names.feature_names
      new_feature = "new"
      new_idx = feature_names.add_feature(new_feature)
      assert len(feature_names) == len_before + 1
      assert feature_names[new_idx] == new_feature
      if storage is not None:
          assert storage is not feature_names.feature_names
  
file_path: eli5/tests/test_feature_names.py
imports: import pytest
  import numpy
  from eli5._feature_names import FeatureNames
label_desc: Use image node_id 5 for calling a global function with example usage: test_add_feature(feature_names) without return types