We are wondering if we could use the user name to get it instead of the user id? If I do get_application_credential(user='catherine', application_credential = 'app_cred_id'), then it will show me an error that "You are not authorized to perform the requested action: identity:get_application_credential". Is there any method that no need user info, can just use the application credential id to get the expiration date? We also didn't find any documentation about the application credential in openstacksdk identity service docs.

You can use:

user = conn.identity.find_user(name_or_id = ‘my_user’)

ac = conn.identity.find_application_credential(user=user, name_or_id=‘app_cred’)