Question

I am very new to TDD. I am reading TDD By Example and it says “never try to use the same constant to mean more than one thing” and it show an example of Plus() method.

In my opinion, there is no difference between Plus(1, 1) which uses same constant value and Plus(1, 2). I want to know what are pros and cons of using same constant value in test method?

Answer 1

I think you misinterprete that statement. What the author (imho) is trying to convey is that following code is a recipe for disaster.

const SomeRandomValue = 32;
...
// Plus testcase
Plus(SomeRandomValue, SomeRandomValue)
...
// Divide testcase
Divide(SomeRandomValue, SomeRandomValue)

You have two testcases reusing a none descriptive constant. There is no way to know that by changing SomeRandomValue to 0 your testsuite will fail.

A better naming would be something like

const AdditionValue = 32;
const DivisorValue = 32;
...
// Plus testcase
Plus(AdditionValue, AdditionValue)
...
// Divide testcase
Divide(DivisorValue, DivisorValue)

where it should be obvious as to what the constants are used for.You should not get to hung up on the idea of code reuse when creating testcases.

Or to put it in other words:

I don’t see anything wrong with reusing the DivisorValue constant in multiple testcases > but there is definitly something wrong trying to shoehorn one value into a none descriptive variable just in the name of code reuse.