How to set the path of stdout_logfile same as the directory?

We know the string expressions from [program-x-section-values](http://supervisord.org/configuration.html#program-x-section-values)

How to set the path of stdout_logfile same as the directory like this?

```shell
[program:echo_time]
command=sh echo_time.sh
directory=/tmp
stdout_logfile=%(directory)s/echo_time.log
```