Better session logic.

flask/sessions.py

@@ -169,9 +169,10 @@ class SecureCookieSessionInterface(SessionInterface):
     def save_session(self, app, session, response):
         expires = self.get_expiration_time(app, session)
         domain = self.get_cookie_domain(app)
-        if session.modified and not session:
+        if not session:
+            if session.modified:
                 response.delete_cookie(app.session_cookie_name,
                                        domain=domain)
-        else:
+            return
         session.save_cookie(response, app.session_cookie_name,
                             expires=expires, httponly=True, domain=domain)