def METHOD_NAME(self, default_config, custom_conf_dir, monkeypatch):
    """Verify that an exception is raised if an ENV variable is to be used in a
    config file, but that variable is not present in the environment.
    """
    default_dir = default_config.parent
    aux_conf = default_dir / "auxiliary.conf"
    write_config(
        aux_conf,
        """
        [foo]
        bar = %(ENV:IDONTEXIST)s
        """,
    )
    with pytest.raises(configparser.InterpolationMissingOptionError):
        _ = Config("auxiliary")