code: def _append_constant_features(features, mapping):
      """Appends constant features."""
      output = dict(features)
      output.update(mapping)
      return output
  
imports: from __future__ import annotations
  import functools
  import json
  import os
  from typing import Mapping
  import numpy
  import tensorflow_datasets.public_api
label_desc: Use image node_id 3 for calling a global function with example usage: _append_constant_features(features, mapping) and returns: output