Why write this test?

Why should I write an automated test?

• to give myself a way to play with the code now
• to provide an easy way to investigate the code in the future
• to suss out any mistakes in a small knot of complex logic
• to force myself to think deeply about the code under test - only a test can cause one to continue looking at code past the point of boredom, which is necessary to find the nuanced bugs
• to continue asserting correct behavior long after one forgets about the code (and after leaving the project)
• to provide a dynamic documentation on intention
• to allow early performance testing (loop it a million times and see how long it takes)
• to sharpen my thinking about how this program will be used, how it could go wrong
• to force the contract to be as simple as possible. If it is hard to test, it will be hard to maintain and debug as well.

When might I not write an automated test?

• it will interfere with my flow and I cannot think of a quick way to get a test on it. Especially so when I'm working on a very unknown domain and/or technology.
• I am working with a complex stateful machine with a difficult interface, and the time to write a proper test would be exorbitant. Examples: database procedures, some fiddly web browser behaviors.
• when the coding is purely cosmetic or prose
• when I have sufficient initial tests.
• when refactoring code that has good tests

Believe it or not, there is a moderation to be had in testing - each new test carries weight and requires maintenance. The goal is to have the fewest tests with the best results.