Returns *conf_name* settings if provided in *all_config*, else returns
     *default_value*.

    Validates *conf_name* value if provided.

def compute_optional_conf(conf_name, default_value, **all_config):
    """Returns *conf_name* settings if provided in *all_config*, else returns
     *default_value*.

    Validates *conf_name* value if provided.
    """
    conf_value = all_config.get(conf_name)
    if conf_value is not None:
        # Validate configuration value.
        conf_value = get_validator(conf_name)(conf_value)
    else:
        conf_value = default_value
    return conf_value