Test Driven Development initial implementation

Question

A common practice of TDD is that you make tiny steps. But one thing which is bugging me is something I've seen a few people do, where by they just hardcode values/options, and then refactor later to make it work properly. For example…

describe Calculator
  it should multiply
    assert Calculator.multiply(4, 2) == 8

Then you do the least possible to make it pass:

class Calculator
  def self.multiply(a, b)
    return 8

And it does!

Why do people do this? Is it to ensure they're actually implementing the method in the right class or something? Cause it just seems like a sure-fire way to introduce bugs and give false-confidence if you forget something. Is it a good practice?

Answer 1

This practice is known as "Fake it 'til you make it." In other words, put fake implementations in until such time as it becomes simpler to put in a real implementation. You ask why we do this.

I do this for a number of reasons. One is simply to ensure that my test is being run. It's possible to be configured wrong so that when I hit my magic "run tests" key I'm actually not running the tests I think I'm running. If I press the button and it's red, then put in the fake implementation and it's green, I know I'm really running my tests.

Another reason for this practice is to keep a quick red/green/refactor rhythm going. That is the heartbeat that drives TDD, and it's important that it have a quick cycle. Important so you feel the progress, important so you know where you're at. Some problems (not this one, obviously) can't be solved in a quick heartbeat, but we must advance on them in a heartbeat. Fake it 'til you make it is a way to ensure that timely progress. See also flow.

Answer 2

There is a school of thought, which can be useful in training programmers to use TDD, that says you should not have any lines of source code that were not originally part of a unit test. By first coding the algorithm that passes the test into the test, you verify that your core logic works. Then, you refactor it out into something your production code can use, and write integration tests to define the interaction and thus the object structure containing this logic.

Also, religious TDD adherence would tell you that there should be no logic coded that a requirement, verified by an assertion in a unit test, does not specifically state. Case in point; at this time, the only test for multiplication in the system is asserting that the answer must be 8. So, at this time, the answer is ALWAYS 8, because the requirements tell you nothing different.

This seems very strict, and in the context of a simple case like this, nonsensical; to verify correct functionality in the general case, you would need an infinite number of unit tests, when you as an intelligent human being "know" how multiplication is supposed to work and could easily set up a test that generated and tested a multiplication table up to some limit that would make you confident it would work in all necessary cases. However, in more complex scenarios with more involved algorithms, this becomes a useful study in the benefits of YAGNI. If the requirement states that you need to be able to save record A to the DB, and the ability to save record B is omitted, then you must conclude "you ain't gonna need" the ability to save record B, until a requirement comes in that states this. If you implement the ability to save record B before you know you need to, then if it turns out you never need to then you have wasted time and effort building that into the system; you have code with no business purpose, that regardless can still "break" your system and thus requires maintenance.

Even in the simpler cases, you may end up coding more than you need if you code beyond requirements that you "know" are too light or specific. Let's say you were implementing some sort of parser for string codes. The requirements state that the string code "AA" = 1, and "AB" = 2, and that's the limit of the requirements. But, you know the full library of codes in this system include 20 others, so you include logic and tests that parse the full library. You go back the the client, expecting your payment for time and materials, and the client says "we didn't ask for that; we only ever use the two codes we specified in the tests, so we're not paying you for the extra work". And they would be exactly right; you've technically tried to bilk them by charging for code they didn't ask for and don't need.