title: Django - set expiry for a certain key in session
body: <p>I'm creating a Django application which allows visitors to vote on a list once every week. My app does not have a User model, so what I'm trying to do is set the session for a specific key to <code>True</code> to signify that the user has voted on that list. eg.</p>
  
  <pre><code>request.session['a'] = True
  request.session.set_expiry(604800)
  </code></pre>
  
  <p>This works fine but the session expiry is set for the entire session and not just that key. How can I set expiry for a specific key in a Django session?</p>