How to load yaml string placeholders for python?

I have a configuration file as config.yaml, that look like this

misc.application_title: "ATT"

data_dir: "${APP_DIR}/aux_data" 

I have a function in app.py where it loads the config from yaml file. My question is how to load ${APP_DIR} as placeholder in the string. I have to load string placeholders while loading yaml file itself. Any leads will be appreciated .