Unit testing, you’re doing it wrong

15 min readOct 19, 2018

TLDR; Existence of untested code in the wild should worry you: most of our lives is software controlled in a way or another. Good news is that you can do something about it. Bad news you may be doing it wrong.


I understand that I am addressing a very sensitive topic; I will probably offend many readers that will say that I am an insane troll and my views are bullshit. Offending is not my objective, but I stand by my opinions. Of course comments are here for you to voice your opinion. And yes this piece is biased by my past experiences, but that’s the value of it, sharing my experiences.

‘How legitimate are you?’’

Fair question. I have a 35 years long career in IT; I have worked at companies of various sizes and culture. I have often been in some transversal position and had the opportunity to meet and work with a lot of developers (think thousands). While most of my roles involved code, I also touched on QA and BA activities. I am now in CTO-like positions for 2500 ITs and had the great privilege to work with well-known french experts, as well as lesser-known ones.

So my point of view is based on things and events I have experienced first-hand as a developer, things I have seen others struggle or succeed with, problems encountered by teams I have helped and views and issues that other experts taught me about. Basically, I have been through all this sh*t and made most of the mistakes I list here. Of course, this does not demonstrate how right I am, but at least, please agree that I have a comprehensive view of what I am talking about.

Some fallacies about unit testing

1. TDD is all about unit tests

Unit tests, really?

Big NO, TDD, a.k.a ‘Test First Development’ is about defining what the code is expected to produce, capturing this as some test(s) and then implementing just enough code to make it pass. Unit testing is understood as testing small parts of the code in isolation, e.g. testing some class’s methods, maybe using some stubs/mocks to strip dependencies.

Unit tests

Unit tests are promoted for their speed and focus: they are small, with limited dependencies, hence run (usually) fast. When a unit test fails, it is easy to identify which part of the code is responsible (for the problem).

TDD is actually about every form of tests. For example, I often write performance tests as part of my TDD routine; end-to-end tests as well. Furthermore, this is about behaviours, not implementation: you write a new test when you need to fulfill a requirement. You do not write a test when you need to code a new class or a new method. Subtle, but important nuance. For example, you should not write a new test just because you refactored the code. If you have to, it means you were not really doing TDD.

And when Kent Beck wrote about tests being isolated, he meant between one and another. For example, having one test inserting records in a table while another reads that same table is probably a bad idea, as the result of the tests may vary depending in the order of which the tests are run.

2. Automated testing is all about unit tests

No, automated testing describes a process: having tests automatically run as part of your build/delivery chain. It covers every kind of tests you can perform automatically: behavior tests, stress tests, performance tests, integration tests, system tests, UI tests….

There is an emphasis on unit tests because they are fast, localized and you can execute them ‘en masse’. But feature tests, use case tests, system tests, performance tests, you-name-it-tests, must also be part of your building chain. You must reduce manual tests as much as you can. Because they are expensive and give slow feedback.

Having a behavior test would help

There is one testing activity that must be done manually (for now, at least): exploratory testing. But this a different activity.

3. 100% code coverage requires extensive unit testing

NO, NO, NO and f…g no. In a perfect TDD world, untested code does not exist in the first place.

Writing a test is akin to writing down a contract or a specification, it freezes and enforces many decisions. Your tests must focus on behavior; behavior driven and use cases tests are the most important ones. Code coverage must consider every tests run, disregarding their type.

Yep, let just add another stupid test

Improving coverage by adding specific tests for untested methods and classes is wrong, wrong, wrong. Additional tests must be justified by some requirements (new or existing); otherwise, it means the code has no actual use. Then bad things will happen: it will drive your codebase to excessive coupling between tests and implementation and tests will break whenever refactoring occurs.

And if the code has actual use, it should be covered by some tests, don’t you think?

For example, if you implement a calendar module that supports Gregorian to Julian conversion, either you have a relevant test for this feature, or you just remove it.

4. You have to make private methods public to reach 100% coverage

I need to assess the private parts

Again, no!; private methods will be tested through public entry points. Once again, unit testing is not about testing every method in isolation.

Wondering about how to test private methods is a clear sign you’ve got TDD wrong. If this is not clear to you, I suggest you stop UNIT TESTING altogether and focus on BDD. When you get the grasp on BDD, will you be able to embrace TDD.

If part of the code cannot be tested in full, you need to challenge the relevance of the non covered part: it is probably useless code.

5. Some code does not need be tested

The design of the Death Star is Rebel proof, right ?!

This one is somewhat true, but probably not to the extent you think it is: code that works by construction does not require testing if it never changes. That being said, please show me code that will never ever change, I have a couple of bridges to sell.

Also, I am only an average developer, and my experience have taught me that my code working on the first attempt is an happy accident.

And even if you were the god of code, chances are somebody else will break your code in a couple of months, weeks or even hours. And yes, that somebody else is probably the future you.

Remember as I said earlier, a test is a contract. And contracts exist because people change, context changes, etc….

What about trivial code? I often get this remark: “Testing getters or setters is simply a waste of time.”. Seems pretty obvious, isn’t it? What is wrong with this remark is the implicit notion of testing (trivial) getters or setters in isolation. Which would probably be not only useless but likely harmful. Unit testing is not about testing method in isolation. Your getters and setters should be tested as part of a larger, behavior related, test (see item #4).

6. You need to use a mocking framework

Look what I have built !

Nope, chances are you don’t.

Mocking frameworks are great pieces of engineering, but almost every time I have seen a team using them, mocks were pervasive within the test base with little to no added value. I have seen tests that ultimately test no production code whatsoever, but it took me hours peering at the code to come to that conclusion.

Often teams are using mocks to test classes in isolation, mocking every dependencies. You dont’ need to do that! Remember, ‘unit’ in unit testing is to be understood as a module or a component, not a class.

Whenever you decide to introduce a mock, you enforce a contract that makes refactoring more difficult. Mocks are here to help you get rid of slow or unstable dependencies, such as a remote services, or some persistent storage.

You should not test for collaboration/dependencies between classes. Those tests are useful if you do bottom-up/inside-out TDD, but you must get rid of them once the feature is complete. Philippe Bourgau has a great set of posts on this topic if you are wanting to dig further.

Oh, I am afraid to say that I am not a fan of the ‘London school’ TDD.

7. Tests are expensive to write

Yes, testing is expensive in most of industries: think about testing a home appliance, a drug or a new car…

This test is expensive!

But code is incredibly cheap, giving the impression that tests are needlessly costly, in a relative way.

They do require extra effort, but they are efficient complement or even replacement for specifications, they improve quality, bring fast feedback, secure knowledge for newcomers…

But ultimately, most automated tests are cheap to write!

On the other hand, green tests look useless both to the team and to management, reducing the perceived value of tests.

8. The ‘testing pyramid’ is the ultimate testing strategy

You probably have heard of the testing pyramid. Basically, it describes that you should have a lot of unit tests, less component tests, then less integration tests, and so on, up to the top of the pyramid where you have a few use case based/acceptance tests. It is used as the default testing strategy for most projects.

Pyramids can be dangerous!

Truth to be told, the testing pyramid outlived its usefulness. Its original purpose was to address the fact that high level tests can have a long execution time and that causes of failure may be hard to identify. It therefore pushes to invest more in unit tests, which are both fast and local, by definition.

This is mostly a dangerous analogy, giving the impression that a ratio of 1000 to 1 between unit and use case based tests is a desirable thing.

You should focus on the top of the pyramid, not the bottom !

I often see teams that have only a couple of high level tests, that cover some of the core use cases, nothing more than glorified smoke tests. And then thousands of ‘method tests’ to ensure a high coverage.

This is bad.

You need to have a decent set of use case based tests for your system, ideally covering all use cases, but handling major ones is a good start. These tests must rely on your high level public APIs, just beneath the user interface. Then have some performance tests for the performance sensitive parts of the application; add to the mix failures reproducing tests, such as external dependencies that are down (using mock to simulate), to make sure your system handles those properly. And then, unit (as in module) tests for the dynamic part of your code base. Then understand the trade off:

  • Having a few unit tests means your design can easily be changed, but it means that finding the root cause of a failing high level tests will take time (and probably involve debugging).
  • Having a lot of those means you find issues as soon as they are introduced in the code base, but significant re-designs of your solution will be ridden with failing tests.

if at any point in time you need to have finer tests, such as class or method tests, throw them away as soon as you no longer need them, such as when the initial design and implementation phase is over. Otherwise they will drag your productivity slowly to the ground.

What about some truths ?

1. Unit tests are not about testing a method in isolation

Here is what Wikipedia proposes:

In computer programming, unit testing is a software testing method by which individual units of source code, sets of one or more computer program modules together with associated control data, usage procedures, and operating procedures, are tested to determine whether they are fit for use.[1]


Good tests must test a behavior in isolation to other tests. Calling them unit, system or integration has no relevance to this.

Kent Beck says this much better than I would ever do.

From this perspective, the integration/unit test frontier is a frontier of design, not of tools or frameworks or how long tests run or how many lines of code we wrote get executed while running the test.

Kent Beck

2. 100% coverage does not mean your code is bug free

This the first rebuttal I get whenever I talk about 100% coverage. Of course, it does not.

Coverage only shows which part of the code have been executed. It does not guarantee that it will work in all circumstances, and it may still fail for specific parameters’ values, some application state or due to concurrency issue. Also, it does not prove the code produce the required output in itself; you need to have adequate assertions to that effect.

This is deep

This is especially true if you only rely on unit testing!

Coverage metrics are not about what is covered, but about what is not covered. Non covered means not tested. So at least make sure that non tested parts are non critical and that important part of your code are properly tested!

3. There is a tooling problem

The truth is unit tests are in the spotlight mostly thanks to tooling! We should be all eternally grateful to Kent Beck for creating sUnit, the library which triggered a testing revolution, but we must not stop there.

Are you using test coverage tools (JCov, Clover, NCover, Jasmine…)? Do you look at their report? Do they influence your decision regarding the tests you need to write?

Have you tried continuous testing tools (InfinyTest, NCrunch, Wallaby…)?

I have a bias: I am addicted to NCrunch.

Having your tests running continuously is a game changer for TDD!


Seriously: DO it, now! It will change your perceived value of tests.

Have you tried Cucumber to have a more use case driven approach?

You may also consider using Mutation Testing , to assess the quality of your tests.

Property Based Testing is useful to check for invariants and higher level abstractions.

Put some time aside and learn a new testing tool, you will be thankful for that in a few months.

4. It is difficult

Yes, but this is no more difficult than designing the software up front. You face complexity, but what is interesting in test first approaches, is that you have an opportunity to focus on essential complexity as test code ought to be simpler than actual implementation.

Tests are hard to write

This is the major challenge when one learns TDD. We rely so much of up front design, than we think about the implementation when we should be thinking about the test.

I have animated many craftsmanship discovering sessions based on Lego exercises (French deck) . After the TDD exercise, attendants often express that the difficult part was choosing the right test, and building the solution was straightforward. Interestingly, even non coder profiles (BA, managers, CxO, …) share this feeling, sometime event saying how comfortable it was just to follow requirements, versus the hardship of identifying a test (in TDD mode).

Choosing the next test is an act of design.

(attributed to) Kent Beck

I attribute this difficulty to a set of factors:

  1. it forces you to think problem first, while solution first is everyone comfort zone
  2. it constrains your design, and nobody likes extra constraints
  3. it gives you the impression of being unproductive

But all those factors turn into benefits:

  1. Problem first is the right focus!
  2. Constraints help you drive the design. And as you are problem first, this is bound to be a good design.
  3. Worst case, tests will be thrown away. But they helped you build a solution and a deep understanding of the problem. At best, they prevent future regression, and provide help and documentation for future developers.

Writing tests is never unproductive.

5. Tests require maintenance

Maintenance is an ongoing effort

Tests require as much maintenance effort as any other piece of code. They need refactoring along the source code; they may also requires refactoring on their own. Tests will have to be updated if new use cases are identified, existing ones must be altered, or if, god forbid, regression(s) appear.

To sum it up: tests are part of your codebase and must be treated as such. Which leads to the next truth:

6. Having too many tests is a problem

Since tests need to evolve with the production code, too much tests will hamper your productivity: if changing some lines of code break hundred tests or more, the cost (of change) becomes an issue.

This is a sure sign of failing to care for your tests appropriately: may be some tests are replicated with only minor variations, each one adding marginal value.

I have seen projects and teams that were grounded to a halt due to having a far too large test base.

Then, more often than not, the team simply throw the test code base away, or cut through it savagely.

Too much tests…

Finaly, tests also increase build time, and as you are doing continuous build/delivery (you are, aren’t you?), you need to keep build time as low as possible, otherwise your feedback delay increases too much.

This leas to next important truth:

7. Throwing away tests is a hygienic move

It should be obvious by now that you need to keep a manageable number of tests. Therefore you must have some form of optimization strategy for you test base. Articles are pretty much non existent for this kind of activity, so let me make a proposal:

  • getting rid of scaffolding tests should be part of your TDD/BDD coding cycle. By scaffolding tests, I mean tests that you used to write the code in the first place, identify algorithm(s) and explore the problem space. Only keep use case based tests.
  • make regular code coverage review, identify highly tested lines and remove tests you find redundant.

You can see this thread for an extensive discussion on having too many tests.


8. Automated tests are useful

Last but not least. Automated tests have a lot of value. Yes, a green test looks useless, like any security device: safety belt, life vest, emergency brakes…

If you practice TDD, tests have value right now. But even if you don’t, tests have value in the long run.

An interesting and important 2014 study analyzed 198 user reported issues on distributed systems (incl HBase, Cassandra, Zookeeper). Among several important findings, it concluded that 77% of the analyzed production issues could have been reproduced by a unit test.

Another key finding was that almost all catastrophic failures were the result of incorrect error handling.

Incorrect error handling

Simple testing can prevent most critical failures

Source study


First of all, thanks to you for having the patience of reading this so far. I’d also like to thanks my reviewers for their feedback, namely: Alexandre Victoor, @MorganLeroi, Thomas Pierrain and yannick grenzinger!

Now, if you are dubious about unit tests, I hope this article cleared some of your concerns and gave you some reason to try using them.

If you are already doing unit testing, I hope I offered you some guidance to help you avoid the dangerous mines that lie ahead. And if you think you’re a master at unit testing, I hope you share my point of views and that I gave you strong arguments to convince other.

Each of the facts I listed previously is worthy of a dedicated talk or article but digging further is left as an exercise for the so minded reader.


  1. Tests are useful, they can prevent catastrophic failures.
  2. Test behaviours, not implementation. A.k.a. understand what unit stands for in unit tests.
  3. Maintain your test base with the delicate but strong hand of the gardener: gently refactoring when necessary and pruning out when no longer useful.

I suggest you also read James Coplien paper (Why Most Unit Testing is Waste). Here are his advices:

Keep regression tests around for up to a year — but most of those will be system-level tests rather than unit tests.

Keep unit tests that test key algorithms for which there is a broad, formal, independent oracle of correctness, and for which there is ascribable business value.

Except for the preceding case, if X has business value and you can test X with either a system test or a unit test, use a system test — context is everything.

Design a test with more care than you design the code.

Turn most unit tests into assertions.

Throw away tests that haven’t failed in a year.

Testing can’t replace good development: a high test failure rate suggests you should shorten development intervals, perhaps radically, and make sure your architecture and design regimens have teeth

If you find that individual functions being tested are trivial, double-check the way you incentivise developers’ performance. Rewarding coverage or other meaningless metrics can lead to rapid architecture decay.

Be humble about what tests can achieve. Tests don’t improve quality: developers do.

Questions, remarks and feedbacks are welcomed!




Cyrille Dupuydauby. Convinced crafter , NewCrafts and Devoxx speaker, OSS maintainer, coding architect. Founder of the Speaker Academy