Skip to main content

Learning Tests

At the last Agile Staffordshire I attended the task was to complete the string calculator with constraints. The group worked in pairs and everything was running smoothly. Until I heard a few guys behind struggling with something.

I'd worked with one of the developers previously, so they called me over to take a look. What he found was pretty shocking - they had found a bug in the .NET framework. The string class of all things. Bugs exist in all code. Bugs in the substring method though are probably rarer given how exhaustively used this particular bit of code is.

The problem was how they expected the method to behave. When creating a substring they were getting confused with how the offests worked.

This is an easy mistake. Different languages or frameworks can have different methods to do similar tasks. I take no shame in not knowing of the top of my head whether the offest of the substring method is an offset of the index, or an offset from the start of the string.

I managed to spot the issue very quickly but never let on. Instead I decided to share a technique which I use regularly to great effect.

Rather than painfully using the debugger and stepping through the code line by line I suggested he write a simple test around the single line of code they were convinced was misbehaving.

After a couple of more tests it was clear how the substring method worked in .NET. Once this was cleared up, we deleted the tests we just wrote and modified the production code to use the correct offset. This whole process took less than sixty seconds.

I explained afterwards that such a technique of writing learning tests or scaffholding tests is incredibly valuable. The feedback cycle here is very quick. Quicker than explaining to another developer what is wrong; Quicker than "Googling" the problem; Quicker than looking at the reference implementation and certainly quicker than using the debugger.

My rules are pretty explicit when dealing with learning tests. They should be short lived. Testing implementation details is often a bad idea, but that is the whole point of such style of testing. Therefore if you do decided to check these tests in tagging them so they are only run as part of CI builds is worthwhile. In other words, just like real world scaffholding, they are temporary. Don't feel bad about writing some tests, only to delete them minutes later.

Learning tests have another nice side effect. They give static languages which have a slower feedback cycle a form of REPL. It's a lot quicker to write a test method and execute than it would be to spin up a new project in languages such as C# or Java to just try something out.

Next time you're stuck, try writing a test.


Popular posts from this blog

Constant Object Anti Pattern

Most constants are used to remove magic numbers or variables that lack context. A classic example would be code littered with the number 7. What does this refer to exactly? If this was replaced with DaysInWeek or similar, much clarity is provided. You can determine that code performing offsets would be adding days, rather than a mysterious number seven.Sadly a common pattern which uses constants is the use of a single constant file or object. The beauty of constants is clarity, and the obvious fact such variables are fixed. When a constant container is used, constants are simply lumped together. These can grow in size and often become a dumping ground for all values within the application.A disadvantage of this pattern is the actual value is hidden. While a friendly variable name is great, there will come a time where you will want to know the actual value. This forces you to navigate, if only to peek at the value within the constant object. A solution is to simple perform a refactor …

Three Steps to Code Quality via TDD

Common complaints and problems that I've both encountered and hear other developers raise when it comes to the practice of Test Driven Development are: Impossible to refactor without all the tests breakingMinor changes require hours of changes to test codeTest setup is huge, slow to write and difficult to understandThe use of test doubles (mocks, stubs and fakes is confusing)Over the next three posts I will demonstrate three easy steps that can resolve the problems above. In turn this will allow developers to gain one of the benefits that TDD promises - the ability to refactor your code mercifully in order to improve code quality.StepsStop Making Everything PublicLimit the Amount of Dependencies you Use A Unit is Not Always a Method or ClassCode quality is a tricky subject and highly subjective, however if you follow the three guidelines above you should have the ability to radically change implementation details and therefore improve code quality when needed.

DRY vs DAMP in Tests

In the previous post I mentioned that duplication in tests is not always bad. Sometimes duplication becomes a problem. Tests can become large or virtually identically excluding a few lines. Changes to these tests can take a while and increase the maintenance overhead. At this point, DRY violations need to be resolved.SolutionsTest HelpersA common solution is to extract common functionality into setup methods or other helper utilities. While this will remove and reduce duplication this can make tests a bit harder to read as the test is now split amongst unrelated components. There is a limit to how useful such extractions can help as each test may need to do something slightly differently.DAMP - Descriptive and Meaningful PhrasesDescriptive and Meaningful Phrases is the alter ego of DRY. DAMP tests often use the builder pattern to construct the System Under Test. This allows calls to be chained in a fluent API style, similar to the Page Object Pattern. Internally the implementation wil…