Object Mentor Blog: Testing Will Challenge Your Conventions

Testing Will Challenge Your Conventions

tottinger — Tue, 17 Jul 2007 22:28:00 -0500

If you are doing test-first development, you are likely to find your old coding conventions are no longer valid. There are a few changes you will need to make to your coding standards and practices.

1. Interfaces suddenly seem like a really good idea when you have to start introducing test doubles of various flavors (mocks, etc). You will find yourself creating interfaces in places where you probably would not have created them before. You’ve always known that you shouldn’t depend or derive from concrete classes, and now you are feeling the pain of non-trivial concrete classes. You are forced, more or less, to comply with the Dependency Inversion Principle. Abstraction becomes a way of life.

2. Singletons and static methods no longer seem like a great way to do work (esp in C++) because you can’t easily isolate a module that makes use of them. The exception is when a static method is totally self-contained and testable, and does not use resources like a database, file system, etc. Only the very trivial can be static. Test doubles matter that much. Suddenly substitutability is a primary measure of code goodness.

3. Private makes less sense than it used to. You can’t test anything that’s private. You need to have ways to sense that your tests are working as intended, and you have to be able to test any method that is interesting. That means less private and more accessors. Get used to living in a more public world. If you need to hide something from users, don’t include it in the interface or abstract base class. “Implements”/”public inheritance” is the new “private”.

4. You need to be able to pass a class everything it might need at construction time, so that you can pack it a stubbed logger and a faked database access module. Otherwise, you will not have the isolation you need to write good unit tests. Calling a concrete class constructor inside the body of a method now makes you cringe, because you realize you’ve missed an opportunity for abstraction, and limited the isolation of your method. The fat constructor argument list is a concession to the need for isolation and testing with mocks (and fakes, and stubs).

5. Smaller methods are the norm. It is hard to test a two-hundred line method. It’s far harder than testing a many smaller, equivalent methods. Whatever is easier to test is easier to write. Large is bad. This has always been so, since optimizers love simple functions, but now it’s real to you as well.

6. You hate inheriting code without tests, because you don’t want to reverse-engineer what the other programmer was thinking. In fact, you find yourself looking at tests before you look at code, because you know the tests are better than comments. Testing becomes a kind of “code clarity” mechanism. The profound bit is that code is more clear if it is more testable, even if you have to “pollute” the class to make it testable by moving variables and methods out of the private space or by creating interfaces, or by using fat constructor argument lists. Testability is the new legibility.

7. Hard-to-use class interfaces are now hard for you to use, not just hard for other people. You have to write the tests, so you have to use the class. Usability determines how much you will like your own code, not cleverness.

8. Performance management by old wives tales is dead. You will build for testability, and then use measurement tools to improve performance. This is what the wizened greybeards have been telling us for a lot of years, and with TDD you start listening. You stop avoiding trivial issues like copying PODS and virtual dispatches. You pay attention to more important issues. With optimization, YAGNI applies until it doesn’t, and when it doesn’t you need to know why and where.

9. You care deeply how long it takes to run the tests because you need to be able to run all of the tests all the time, after only a few lines of code are written, and after each refactoring. If it’s taking more than 20 or 30 seconds to run all your unit tests, you start looking for ways to speed it up. You must maintain test performance, because it is key to productivity.

10. Dependency hurts. You can’t afford ‘god classes’ and ‘global hubs’. If test setup becomes a chore, you immediately start reaching for the axe. You have to keep your modules isolated better than ever before. Of course, you should have done this all along, but you never felt it this keenly.

11. “Clever” is dead. Clever is hard to refactor. Clever is hard to isolate, hard to internalize, hard to phrase in tests. One point of “obvious” is worth two hundred points of “clever”.

12. Your IDE is good to the extent that it allows you to do quick write/build/test cycles. You need to do several per minute.

For the most part, TDD forces you to start doing the things you always should have done, though it signals a change in values that should be reflected in your coding standards.

Maybe you should get a highlighter and mark each section of your coding standard or architectural document that encourages any practice that in turn discourages testability. The world has changed, values have shifted, and your style guide needs to change if it is to remain relevant.

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