In test driven development, do you write every possible test first, then the code?

Question

In doing test driven development I have been in the habit of writing the first unit test for a new piece of functionality first, then writing the code for that functionality. If I have additional tests to write to cover all scenarios, I usually write them after the code is written. Is this considered bad form? Should I try and write every conceivable test for a piece of functionality first, before ever writing that code?

Answer 1

In order to do TDD properly, you always write the test first, and then the functionality second.

To add to that, I would take one scenario at a time, don't write 20 tests and then write the code for those 20 tests. Write one test, red/green flag it, then move on to your next test. This makes sure you're also doing one of the core tenants of TDD, which is to do the simplest implementation possible that meets all of your requirements/scenarios.

Answer 2

actually no, I often discover functionality "on-the-go". Let me explain the "no" a bit further:

I usually start out writing a test case for a high level feature, defining its Interface. After that, I usually set this test to ignore and continue writing tests for each of the Interfaces functionality. My cycle goes like:

1. Integration Test for Story A (high level API)
2. Write Unit Test for method xyz called in Integration Test
3. Implement method (red/green/refactor)
4. Repeat 2+3 till Integration Test passes.

While doing so, I often realize I have forgotten some small functionality in my main test. I then usually take time to look back at my customers requirements. If its a fit, I go back and add a test for it, set to ignored as I first want to finish what I started.

Sometimes I see the chance to do a refactoring. I usually finish an implementation till I reach a commit point and do refactoring then, however sometimes I stash my changes, go back and do the refactoring (including new tests if nescessary) first. This workflow is powererd by Mercurial MQ.

Answer 3

The sooner you write the tests, the better. I usually find writing tests being harder tasks than actually implementing the functionality because you have to be aware of all the possible outcomes. So I tend to write more tests when I'm "in the zone". And when during coding I realize I might have missed a test case I just note that down on the to-do lists.

So in my opinion it's up to your leisure but I would implement tests in multiple batches.

Answer 4

For most people, TDD and incremental/agile development go together. This looks something like:

1. Write a test for some feature
2. Write just enough code to make the test pass, refactoring as necessary
3. Repeat.

If you happen to have a detailed specification ahead of time, you could write all of the tests first, but you'd have to live with having sone tests not passing for a while.

Answer 5

The point of TDD is that you have to observe that test fails when feature is not yet implemented. So you have to write test before code.

Answer 6

When you get into the TDD rhythm you write one test at a time and make it work. Very short red-green-refactor cycles really feel the rhythm. That being said, there is nothing wrong with other approaches (and they may even make more sense for some types of problems) but typically the only thing you need to do about other tests you are thinking of is write them down (or have your pair if you are pair programming write them down) so you don't forget them. You have to do that anyway because you could forget about a test in the middle of writing a different test.

Answer 7

The way I see it, test driven development isn't necessarily tests first development. Your tests drive your development and you are really writing your tests as you develop your application. You start by writing a simple test that fails because you haven't written the functionality yet. Then you write your code to implement that so that the tests pass.

Then you go back to your test, make modifications that will force you to add more functionality or refactor your code to follow better practices or reduce duplicate code, go fix your code to make the test pass...repeat, repeat, repeat.

A couple of videos that demonstrates this is below, although you can probably find a lot more by googling "TDD Video"

http://agilesoftwaredevelopment.com/videos/test-driven-development-basic-tutorial (oops, only one video, new users can't insert more than one link)

Answer 8

Do just enough tests to test 1 unit of code at a time.. then write the actual code until it passes the test.. rinse, wash, repeat until done.

If you find yourself needing to write many tests for one unit of code ( a method, a function etc) it might be a sign that you are trying to do too much in that unit... which in turn makes the unit dificult to test & to refactor at a later time.

Answer 9

I try to write a test at some level before each bit of functionality. Sometimes, I have to write a little more code to get through the compiler, but I try to minimise that. Writing the test first means that I've thought about what the code is supposed to achieve before writing it.

One technique I find useful is to keep an index card or notepad handy, and make a note of all the cases that I think of along the way. That allows me to focus on the current task without losing track of all the other things I'm supposed to think about. Afterwards, I can work through the list and either complete the extra cases or drop them as not necessary.

Answer 10

You could do that, but you wouldn't be doing TDD. The problem (well, one of them, anyway) with writing all of your tests up front is that in any case where the requirements are non-trivial, your tests will be building in a lot of assumptions about the structure of the code you're test-driving. Big steps lead to missteps.

One of the keys of successful TDD involves taking small steps. Small steps mean fewer changes to back out when something goes wrong. Small steps mean you can more often get your head around the effects of the changes you're making. And because small steps are easier to take with confidence, they have the paradoxical effect of increasing your velocity.

The TDD cycle starts with requirements. Start by choosing a requirement you know how to define through tests immediately, in a few short steps. If you look at a requirement and you're not sure how to test it, or you think, "Yeah, but to do that, I'd need to [insert ill-defined steps] first", then you should either skip to another requirement that you know how to do, or you should break this requirement into smaller requirements that you know how to do.

Once you have that, you work in a short red-green-refactor cycle: Write a test that quantifies some part of the requirement ("red", because it fails, because it has no implementation to test yet), write any code that will pass the test ("green"), then rework the code to remove duplication, magic numbers, and other code smells ("refactor"). During the refactoring phase, you should continue working in small steps, frequently re-running the test to make sure you haven't broken anything. Continue this cycle until you can look your boss/client in the eye and call the requirement met.

Now that you have one simple piece of your system defined, you've opened up the list of requirements available to implement - requirements that are adjacent to or dependent on the one you just implemented can now be tested and implemented in smaller steps building on what you've already done.

So the upshot of all that is: Don't try to do all your tests at once. One (small) thing at a time.