toy example + unit test by its side

mynumber.py

class MyNumber:
    def __init__(self, n: int):
        self.__n = n

    def __repr__(self) -> str:
        return f"{self.__class__.__name__}({self.value})"

    def __str__(self) -> str:
        return str(self.value)

    @property
    def value(self) -> int:
        return self.__n

    def add(self, delta: int):
        self.__n += delta

    def mul(self, times: int):
        self.__n *= times

    def div(self, by: int):
        self.__n /= by


def main() -> int:
    # this is not printed when this module is imported, or tested below
    print("Main activated!")
    return 0


if __name__ == '__main__':
    raise SystemExit(main())

mynumber_test.py

# Test: python3 -m unittest mynumber_test.py

import unittest
from mynumber import MyNumber


class MyNumberTest(unittest.TestCase):
    def setUp(self):
        # This is triggered before each test
        self.num = MyNumber(0)

    def tearDown(self):
        # This is triggered after each test
        pass

    def testCannotDivideByZero(self):
        with self.assertRaises(ZeroDivisionError):
            self.num.div(0)
            # if we reach here, the test fails

    def testAdd(self):
        self.num.add(1)
        self.num.add(666)
        self.assertEqual(self.num.value, 667)

    def testSubtract(self):
        self.num.add(-2)
        self.assertEqual(self.num.value, -2)

    def testStrValue(self):
        self.assertEqual(str(MyNumber(666)), '666')