code: def environment_variable(key: str) -> EnvironmentVariablePlaceholder:
      """Returns a Placeholder representing EnvironmentVariable proto.
  
      Args:
        key: The key of the environment variable.
  
      Returns:
        A Placeholder that supports
          1. Rendering the value of an environment variable for a given key.
             Example: environment_variable('FOO')
          2. Concatenating with other placeholders or strings.
             Example: 'foo=' + environment_variable('FOO')
      """
      return EnvironmentVariablePlaceholder(key)
  
imports: from __future__ import annotations
  import typing
  from typing import Any, Literal, Optional, Union
  from tfx.dsl.placeholder import placeholder_base
  from tfx.proto.orchestration import placeholder_pb2
  from google.protobuf import message
label_desc: Use image node_id 4 for calling a global function with example usage: environment_variable(key) and returns: EnvironmentVariablePlaceholder