Unit Tests Questioned

2017-01-21, , Comments

I’m looking forward to talking at TechHub Swansea at the end of the month on the subject of unit testing.

The introduction to the talk is here.

Hope to see you there.

Unit Tests Questioned

In this talk we’ll consider if unit testing is a good use of our time.

I am not questioning whether testing is important. This slide shows the abstract of a paper published in 1996 by Tony Hoare. What he says is that you don’t need to prove your software works: you can make it so through design and review; by testing; and by good engineering.

How did software get so reliable without proof?

This may no longer seem controversial or novel but maybe it was a surprising thing for Tony Hoare to say. After all, he invented quicksort back in 1959, an algorithm which he proved correct. He also proved how quickly it runs, and that — in the general case — it couldn’t be beaten. Much of Hoare’s subsequent research was dedicated to proving software works using techniques known as formal methods. The idea is that you write a formal specification of what your program should do, and then you mathematically verify your code satisfies that specification. Surely nothing less is good enough for software which really is a matter of life and death: a life support system, or a device driver in a nuclear warhead, for example? Actually, no, he says in this paper, there are more pragmatic routes to reliable software.

So, the idea that you can build reliability via testing may once have seemed radical. It doesn’t any more. Maybe, though, the conventional modern wisdom that unit tests are a good thing should be questioned.


James O Coplien voices a dissenting opinion in a more recent paper, “Why most unit testing is waste”.

What’s Cope on about!?

Such is the force of current thinking that even the title of this paper makes it seem like he’s trolling. Having read the paper carefully, I don’t think he is. That doesn’t mean I agree with him, but I agree with his attitude:

There’s a lot of advice, but very little of it is backed either by theory, data, or even a model of why you should believe a given piece of advice. Good testing begs skepticism. Be skeptical of yourself: measure, prove, retry. Be skeptical of me for heaven’s sake.

So, this talk will do just that.

Unit tests: let’s get skeptical!