The key to writing applications that are easy to maintain is to write clean and well-structured code. The examples that you have seen so far are too simple to demonstrate this, but Flask view functions have two completely independent purposes disguised as one, which creates a problem.
The obvious task of a view function is to generate a response to a request, as you have seen in the examples shown in Chapter 2. For the simplest requests this is enough, but in many cases a request also triggers a change in the state of the application, and the view function is where this change is generated.
For example, consider a user who is registering a new account on a website. The user types an email address and a password in a web form and clicks the Submit button. On the server, a request with the data provided by the user arrives, and Flask dispatches it to the view function that handles registration requests. This view function needs to talk to the database to get the new user added, and then generate a response to send back to the browser that includes a success or failure message. These two types of tasks are formally called business logic and presentation logic, respectively.
Mixing business and presentation logic leads to code that is hard to understand and maintain. Imagine having to build the HTML code for a large table by concatenating data obtained from the database with the necessary HTML string literals. Moving the presentation logic into templates helps ...