When the session timeout,how can I get the session info by using beaker in bottle

[yasinasama]

I use beaker for session in bottle,and when someone login I recode the username to session.Like:

s = request.environ.get('beaker.session')
s["user"] = "someone"

Then here is the situation I encountered:

I set the session.timeout:1800,that means 30 mins later the session will be timeout.When the session timeout.I want to get the value of s["user"] so that I can do something with this value.

In a word,if I can catch something when beaker.session is ready to timeout.