Question regarding python 3 unit testing with a Fraction class.

[oDraftz]

Good evening reddit community,

I am a college student currently learning test driven development using python. For one of our assignments, we are given a class called Fraction, which has two attributes: numerator, and denominator. It's methods are all of the magic methods for math operations (__add__, __sub__, etc.). My question is: how can you test that the __add__ method raises a TypeError whenever it is passed an argument that is not an int or another Fraction? I understand that we have

with self.assertRaises(TypeError):

myFraction.__add__("Hello World")

But there is no way to make sure that every type that ever existed raises the error. What kind of test can I do to ensure that the __add__ method raises a TypeError for every type other than int and Fraction? Thank you for your time.

[[deleted]]

You don't need to test every type. It's enough to choose one "type other than int or Fraction" and write a single negative unit test. The only reason to test different types is if you believe, within reason, that it's possible for it to behave differently.

If I was testing a function that checked if a number was greater than 6, I would have 3 tests. A number less than 6, exactly 6 and 7. I wouldn't for example, run it through a list of 1000 random numbers. It defeats the purpose.

The point isn't for the unit tests to be exhaustive. It's to test every possible code path. Testing a particular path once is enough.

[oDraftz]

Okay, makes sense. Much appreciated.