Is there an IF-statement in Flask for Python3 that I can use which equates to

I am developing a Flask web app currently. For my login page, I used session['logged_in'] = True to show a user as logged in when they submit the login form. However, if the user closes the window, then reopens it, they stay logged in! Is there an if-statement I could use which equates to the following:

<IF USER CLOSES WEBPAGE>:
    session['logged_in'] = False