Test cases for two functions with different implementations but return same value

Question

I have two functions with different implementations that return the same value, how do I run the same set of test cases for both the functions without violating the DRY principle? for example: I'm implementing recursive and imperative way of implementing factorial of number. I want same set of test cases to run for both the functions. How can I implement without violating DRY?

Answer 1

Your two implementations share a contract that you can express through a set of tests. For example, imagine two functions that double their input:

def double_by_multiplying(value):
    return 2 * value


def double_by_adding(value):
    return value + value

Using one of the methods suggested in abstract test case using python unittest, we can write a general set of tests for any function that doubles its input, then create subclasses that use each of our two implementations:

from unittest import TestCase


class DoublingContract:

    def test_input_gets_doubled(self):
        self.assertEqual(self.implementation(1), 2)


class DoubleMultiplicationTests(DoublingContract, TestCase):

    def setUp(self):
        self.implementation = double_by_multiplying


class DoubleAdditionTests(DoublingContract, TestCase):

    def setUp(self):
        self.implementation = double_by_adding