I'm migrating from C#.NET to Python/Flask. In .NET I have MasterPage, UserControl, PartialView each has its own code behind. e.g. I can save the check login functions in Login.ascx.cs and render the Login.ascx wherever I'd like to. If logged in, it shows the welcome message, else shows the login form. But in Flask I only found {% include 'login.html' %} which include the static html file. How can I implement this design in Flask?