Why should unit tests test only one thing?

Question

What Makes a Good Unit Test? says that a test should test only one thing. What is the benefit from that?

Wouldn't it be better to write a bit bigger tests that test bigger block of code? Investigating a test failure is anyway hard and I don't see help to it from smaller tests.

Edit: The word unit is not that important. Let's say I consider the unit a bit bigger. That is not the issue here. The real question is why make a test or more for all methods as few tests that cover many methods is simpler.

An example: A list class. Why should I make separate tests for addition and removal? A one test that first adds then removes sounds simpler.

Answer 1

Testing only one thing will isolate that one thing and prove whether or not it works. That is the idea with unit testing. Nothing wrong with tests that test more than one thing, but that is generally referred to as integration testing. They both have merits, based on context.

To use an example, if your bedside lamp doesn't turn on, and you replace the bulb and switch the extension cord, you don't know which change fixed the issue. Should have done unit testing, and separated your concerns to isolate the problem.

Update: I read this article and linked articles and I gotta say, I'm shook: https://techbeacon.com/app-dev-testing/no-1-unit-testing-best-practice-stop-doing-it

There is substance here and it gets the mental juices flowing. But I reckon that it jibes with the original sentiment that we should be doing the test that context demands. I suppose I'd just append that to say that we need to get closer to knowing for sure the benefits of different testing on a system and less of a cross-your-fingers approach. Measurments/quantifications and all that good stuff.