code: def test_extension_values():
      config = Config()
  
      # check standard settings
      assert config.root_doc == "index"
  
      # can't override it by add_config_value()
      with pytest.raises(ExtensionError) as excinfo:
          config.add("root_doc", "index", "env", None)
      assert "already present" in str(excinfo.value)
  
      # add a new config value
      config.add("value_from_ext", [], "env", None)
      assert config.value_from_ext == []
  
      # can't override it by add_config_value()
      with pytest.raises(ExtensionError) as excinfo:
          config.add("value_from_ext", [], "env", None)
      assert "already present" in str(excinfo.value)
  
imports: import pickle
  import time
  from pathlib import Path
  from unittest import mock
  import pytest
  import sphinx
  from sphinx.builders.gettext import _gettext_compact_validator
  from sphinx.config import ENUM, Config, _Opt, check_confval_types
  from sphinx.deprecation import RemovedInSphinx90Warning
  from sphinx.errors import ConfigError, ExtensionError, VersionRequirementError
label_desc: Use image node_id 5 for calling a global function with example usage: test_extension_values() without return types