I Hate It When My Tests Pass
I’ve been writing a bunch of unit tests recently, and I came across an odd behavior. I was writing the tests for existing code and whenever I would write a test, as soon as it passed, I wouldn’t trust it. Nothing ever works the first time around. It simply can’t be. So I found myself repeating quite an odd ritual:
- Write a test.
- Run it. It passed. There’s no way that really works…
- What to do, what to do? I know! Let’s change the expected value on the asserts and see what happens!
- The test fails! Yay!
- Change it back. Now I know it works!
At the moment I realized I was doing something weird, I immediately saw all the unit tests I ever wrote flash before my eyes. I was always doing this ritual*. So I’m thinking - what can be done to give me more confidence in my tests? One solution I can think of is a somewhat exaggerated form of verbosity. Take the unittest module for example. Its assert method all take a
msg argument, which is printed in case the assert aborts. Well, how about adding a
success_msg argument? Imagine working on a test, setting the verbosity to a maximum and see the following:
These messages can even be automated, to some degree.
Does this happen to you as well? Do you have a solution to this problem?
*Actually, there is a subset of my tests in which I have very high confidence. Those are the tests I developed with TDD. This post isn’t the right scope for that, but this is one of the advantages of TDD.Discuss this post at the comment section below.
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