I'm Matthew J. Morrison.

A Passionate, professional software developer & hobbyist; Language nerd & regular user of Unix, Python, Ruby & JavaScript.

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Unit testing and test driven development are things that I am very passionate about. Writing good unit tests is not always as straight forward as you would hope and it takes practice to really get good at it. Even then, there are still things that are inherently hard to test.

Inheritance is one area where unit testing becomes difficult. The difficulty does not necessarily come from actually writing the tests themselves (though obviously, some of it may be) but more in figuring out which tests to write, and how to go about doing it.

Say, for instance, I have the following class

class FirstGradeMath(object):

    def __init__(self, first_number, second_number):
        self.first_number = first_number
        self.second_number = second_number

    def add(self):
        return self.first_number + self.second_number

    def subtract(self):
        return self.first_number - self.second_number

with the following tests

import unittest
import first_grade
class FirstGradeMathTests(unittest.TestCase):

    def setUp(self):
        self.sut = first_grade.FirstGradeMath(2, 2) 

    def test_addition(self):
        self.assertEqual(4, self.sut.add())

    def test_subtraction(self):
        self.assertEqual(0, self.sut.subtract())

And I want to extend this class for second graders to include multiplication and division. This is where I run into a dilemma. What is my first test for the second grade math implementation? I could start a number of different ways:

  • I could re-test drive addition and subtraction and refactor by adding FirstGradeMath as a parent class
  • I could copy the tests from FirstGradeMathTests and paste them into SecondGradeMathTests
  • I could assert that SecondGradeMath is a subclass of FirstGradeMath
  • I could Inherit the FirstGradeMath Tests I’d like to review the pros and cons of each of those options.

With the first option, re-test driving addition and subtraction then making SecondGradeMath a subclass of FirstGradeMath seems like a tedious waist of time. This approach will not scale. Can you imagine writing the tests for Algebra, Trigonometry, and Calculus when each time you have to start with addition? However, this is probably the most pure test driven approach. If you prefer purity over pragmatism this may be your preferred approach.

In the second option, copying all of the tests and pasting them is going to be painful. Now you have identical tests that are exactly duplicated. Hopefully, you have some sort of static analysis tool that will let you know that you are sinning if you choose this approach. If, at some point, there are additional tests added for some edge cases the addition tests you will have to remember to duplicate them for FirstGradeMath and SecondGradeMath. This has the same scaling issue as the previous approach. How many tests are you going to have to copy, and how many times? On the positive side, your tests for SecondGradeMath do not make any assumptions about the implementation and assert that addition and subtraction both work correctly in addition to multiplication and division.

import first_grade

class SecondGradeMath(first_grade.FirstGradeMath):

    def multiply(self):
        return self.first_number * self.second_number

    def divide(self):
        return self.first_number / self.second_number

The third option takes a very different approach than the first two. This option makes the assumption that FirstGradeMath is a solid implementation that has been thoroughly tested and is as correct as possible. The problem with this approach is that both the addition and the subtraction methods depend on data that is set in the constructor of FirstGradeMath. If, for some reason, the SecondGradeMath implementation does something differently in the constructor, it could potentially break the implementations of addition and subtraction. This may be a viable solution if you are comfortable making the necessary assumptions. If you control both the parent and the child class implementations, maybe you don’t need to make any assumptions. This approach definitely has the least amount of friction, but has the potential to be risky.

The final option is somewhat of a middle ground between all of the first three approaches. By creating new tests for SecondGradeMath that inherit from the FirstGradeMath Tests you are getting a similar effect of copying the original FirstGradeMath tests. So you are covered from a regression standpoint. At the same time your test code is mirroring the implementation, so since SecondGradeMath extends FirstGradeMath, SecondGradeMathTests extends FirstGradeMathTests.

Lets take a look at what the code would look like for the initial SecondGradeMath tests and implementation with the addition of multiplication and division:

There is one major issue with the tests at this point. They violate one of the FIRST rules of unit testing. (Fast, Isolated, Repeatable, Self-Validating, Timely). These tests aren’t isolated. Each test is dependent upon some instance variable called “sut” being set up prior to the test running, and each test is dependent upon that object’s state. Maybe this is alright if the only thing that we will ever need to add, subtract, multiply or divide is 2 and 2, but that is not the case here.

Inheriting unit tests requires an abstraction, an instance of a *GradeMath, however is not the correct abstraction. If we remove the “sut” variable from the setUp methods and just instantiate FirstGradeMath in the FirstGradeMathTests and SecondGradeMath in the SecondGradeMathTests then inheriting the FirstGradeMath tests are not going to really test anything about SecondGradeMath. Our abstraction level needs to be higher than an instance, so let’s move up one level to the class.

Here are the revised tests.

import first_grade
import second_grade
import unittest
class FirstGradeMathTests(unittest.TestCase):
    sut_class = first_grade.FirstGradeMath

    def test_addition(self):
        sut = self.sut_class(2, 2) 
        self.assertEqual(4, sut.add())

    def test_subtraction(self):
        sut = self.sut_class(2, 2) 
        self.assertEqual(0, sut.subtract())

class SecondGradeMathTests(FirstGradeMathTests):
    sut_class = second_grade.SecondGradeMath

    def test_multiplication(self):
        sut = self.sut_class(2, 2)
        self.assertEqual(4, sut.multiply())

    def test_division(self):
        sut = self.sut_class(2, 2)
        self.assertEqual(1, sut.divide())

This is much better. Now each test class tests exactly one implementation but is free to instantiate that implementation however is necessary for the test at hand. This provides the isolation that the previous tests were missing.

I prefer this method of testing when inheritance is in the picture over most others. In the end, the best approach is most likely different depending on the situation.