When I work with developers who haven’t practiced Test-Driven Development (TDD) one question often arises, When is code too simple to test? As someone who finds an incredible amount of value in TDD and microtesting, my advice is that we should strive to make all of our code simple and still test it.
It’s common for developers to avoid testing objects like adapters or object mappers, for example, thinking they are just too simple to test. They would rather test these objects via inspection than write an automated test, believing that these tests take too long to write or are a burden to maintain.
Since this conversation typically occurs on Test After Development (TAD)1 teams, it usually coincides with conversations about code that is too hard to test. TAD teams commonly attempt to wrap untestable code in automated tests, instead of designing for testability. The net result is a very small amount of functionality that gets covered in complicated tests. I’ve seen these situations result in less than 10% of a team’s code being tested.
TDD practitioners avoid spending time and mental cycles attempting to answer the question; there isn’t anything too simple to test. The test is the mechanism for getting code into the codebase. Without a failing test the code wouldn’t be produced. Once tests exist they rely on refactoring to create simplicity in the design of the production code. TDD practitioners aren’t trying to achieve simplicity instead of writing a test but rather relying on tests to achieve simplicity.
This doesn’t mean there isn’t human judgement in deciding what test cases get written. TDD doesn’t cover every possible test; 100% code coverage is not the goal. Testing every imaginable scenario is not realistic and often results in duplicate testing.
Instead we suggest that developers should test what is interesting. This is intentionally subjective, assuring that human judgement is always a part of TDD and microtesting. This may seem like a subtle difference at first, but simple and interesting are not necessarily opposites. An object mapper can be simple, but if it’s primary responsibility is to map fields from one object to another, I would contend that the mapping is interesting and should be developed by test-driving.
Skilled TDD practitioners create isolated, object-level microtests. The resulting microtest suite contains many tiny tests that run extremely fast (in a few milliseconds) and execute and test a very small path in the code. These tests are favored over larger and more complicated tests. TAD unit testing often results in extraneous tests and tests that are difficult to understand.
Testing simple code in small tests provides error localization. I’ve seen plenty of bugs in simple objects. The tests, while not a silver bullet, give developers an opportunity to ensure that the objects they are building are operating as expected. When a test fails, the line of code containing the problem can be identified very quickly given the small surface area of the tested code.
Simple code often gets more complex over time. As features and other code modifications are added to the system, complexity accumulates. Tests are necessary to support continuous refactoring to avoid this. Assuming that a developer will add tests when they need to refactor discourages continuous refactoring.
When developers raise concerns about the maintainability of tests for simple code, I suggest that simple code often has simple tests. Keeping tests simple is one of the key characteristics of quality microtests that I’ve written about here. Simple tests have an extremely low maintenance cost. So low, in fact, that their value in documenting a fact or behavior of the system exceeds their cost of maintenance.2
Consequently, more complex unit tests with larger test scopes that test across multiple boundaries end up having a much higher maintenance cost.
The documentation rule of microtesting states that you read the test to understand the code it’s testing. You don’t read the code to explain a test. Unfortunately, typical Test After Development unit tests are complicated enough that they don’t provide this level of documentation value.
The tests for simple code carry their weight simply because they are documenting a fact or behavior of the system. Any developer can read a test and understand exactly how the code works.
Instead of spending valuable time and mental cycles trying to figure out when the line has been crossed from “too simple to test” to “complicated enough to test”, drive your development and design with test first development practices. There is value in those really simple tests. Test all of the interesting things you can3 and build testability in from the start.
There are areas of systems that can be hard to microtest depending on the developers’ experience: boundaries of the system, integration points with third-party libraries, code with Awkward Collaborators, etc. There are several strategies to help developers test this code. However, most code that is “too hard to test” is the result of poor design that can be improved to enable testability. ↩