Why you need to create tests 
This is a must-read for our module code developers. Although written for a Django tutorial, the explanation of why you must write test functions for your code is equally valid for people writing in C++ or any other language.
Tests will save you time¶
Up to a certain point, ‘checking that it seems to work’ will be a satisfactory test. In a more sophisticated application, you might have dozens of complex interactions between components.
A change in any of those components could have unexpected consequences on the application’s behavior. Checking that it still ‘seems to work’ could mean running through your code’s functionality with twenty different variations of your test data to make sure you haven’t broken something - not a good use of your time.
That’s especially true when automated tests could do this for you in seconds. If something’s gone wrong, tests will also assist in identifying the code that’s causing the unexpected behavior.
Sometimes it may seem a chore to tear yourself away from your productive, creative programming work to face the unglamorous and unexciting business of writing tests, particularly when you know your code is working properly.
However, the task of writing tests is a lot more fulfilling than spending hours testing your application manually or trying to identify the cause of a newly-introduced problem.
Tests don’t just identify problems, they prevent them¶
It’s a mistake to think of tests merely as a negative aspect of development.
Without tests, the purpose or intended behavior of an application might be rather opaque. Even when it’s your own code, you will sometimes find yourself poking around in it trying to find out what exactly it’s doing.
Tests change that; they light up your code from the inside, and when something goes wrong, they focus light on the part that has gone wrong - even if you hadn’t even realized it had gone wrong.
Tests make your code more attractive¶
You might have created a brilliant piece of software, but you will find that many other developers will refuse to look at it because it lacks tests; without tests, they won’t trust it. Jacob Kaplan-Moss, one of Django’s original developers, says “Code without tests is broken by design.”
That other developers want to see tests in your software before they take it seriously is yet another reason for you to start writing tests.
I particularly appreciate their final reason because it definitely applies to the MUSES collaboration:
Tests help teams work together
The previous points are written from the point of view of a single developer maintaining an application. Complex applications will be maintained by teams. Tests guarantee that colleagues don’t inadvertently break your code (and that you don’t break theirs without knowing).