Write custom assertions whenever possible

I’ve been very interested in readable tests with great error messages recently. Mostly because they kept failing and I wanted the most information possible in order to quickly identify the cause. This is another reason why I like TDD: you see the test failing first, so if the error message isn’t great you’ll know straight away instead of months later.

The good testing frameworks provide a way of writing your own custom assertions. I’d never really looked into them that much before, but now I realize the error of my ways. Recently I wrote a test that contained this line:

fileName.exists.shouldBeTrue;

Readable, right? The problem is when it fails:

foo.d:42 - Expected: true
foo.d:42 -      Got: false

And now you have to go read the test and figure out what went wrong. It’s a lot better to get the information that a file was supposed to exist instead right away. So I wrote a custom assertion and was then ready to write this:

fileName.shouldExist;

With the corresponding failure message:

foo.d:42 - Expected /tmp/foo.txt to exist but it didn't

Now it’s a lot easier to pinpoint where the problem is. For starters, you would probably want to start checking the contents of the surrounding directory, having saved the time you would have had to spend figuring out what exactly was false.

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