code: def _get_env_variable(variable_name: Text) -> Union[Text, None]:
      """Looks up the value of environment varialbe variable_name.
  
      Args:
          variable_name: Name of the variable to look up in the environment.
      Returns:
          A string representing the varialbe value or None if varialbe is not
          defined in the environment.
      """
      return os.getenv(variable_name)
  
imports: import enum
  import json
  import os
  import sys
  import time
  from typing import Dict, Text, Union
  from ..None import version
  from absl import logging
  from googleapiclient import discovery
  from googleapiclient import errors
  from googleapiclient import http
label_desc: Use image node_id 3 for calling a global function with example usage: _get_env_variable(variable_name) and returns: os