Use session.clear() sign-out.

NOTE: you can not test that the session is cleared out by checking the session cookie does not exist on the index page,
because ItsDangerousSession will create a new session when it hits the index page. The unit test confirms that the session has been cleared.

app/main/views/sign_out.py

@@ -8,7 +8,6 @@ from app.main import main
 @main.route('/sign-out', methods=(['GET']))
 @login_required
 def sign_out():
-    if session.get('service_name', None):
-        session.pop('service_name')
+    session.clear()
     logout_user()
     return redirect(url_for('main.index'))

tests/app/main/views/test_sign_out.py

@@ -1,6 +1,4 @@
-from datetime import datetime
 from flask import url_for
-from app.main.dao import users_dao
 
 
 def test_render_sign_out_redirects_to_sign_in(app_):
@@ -24,6 +22,8 @@ def test_sign_out_user(app_,
         email = 'valid@example.gov.uk'
         password = 'val1dPassw0rd!'
         with app_.test_client() as client:
+            with client.session_transaction() as session:
+                print('session: {}'.format(session))
             client.login(api_user_active)
             # Check we are logged in
             response = client.get(
@@ -33,3 +33,4 @@ def test_sign_out_user(app_,
             assert response.status_code == 302
             assert response.location == url_for(
                 'main.index', _external=True)
+            assert session.get('ItsdangerousSession') is None