def set_defaults(conf, executor_thread_pool_size=None):
    """Set defaults for configuration variables.

    Overrides default options values.

    :param conf: Config instance specified to set default options in it. Using
     of instances instead of a global config object prevents conflicts between
     options declaration.
    :type conf: oslo.config.cfg.ConfigOpts instance.

    :keyword executor_thread_pool_size: Size of executor thread pool.
    :type executor_thread_pool_size: int
    :default executor_thread_pool_size: None

    """
    if executor_thread_pool_size is not None:
        conf.set_default('executor_thread_pool_size', executor_thread_pool_size
            )
