code: def _add_feature_names(root, feature_names=None):
      for node in _treeiter(root):
          if not node.is_leaf:
              feat_id = node.feature_id
              if feature_names is None:
                  node.feature_name = "x%s" % feat_id
              else:
                  node.feature_name = feature_names[feat_id]
  
file_path: eli5/eli5/sklearn/treeinspect.py
imports: from __future__ import absolute_import, division
  from sklearn.base import ClassifierMixin
  from sklearn.tree import _tree, export_graphviz
  from eli5.base import TreeInfo, NodeInfo
label_desc: Use image node_id 4 for calling a global function with example usage: _add_feature_names(root, feature_names) without return types