Extensive and well crafted tests are a hall-mark of good software. This article walk through fundamentals of unit-tests in Python using the built-in unittest module.

Basic example

# file: operations.py
def add(a: int, b: int) -> int:
    return a + b


def div(a: int, b: int) -> float:
    if b == 0:
        raise Exception("cannot divide by zero")

    return a / b

# file: operations_test.py
from unittest import TestCase
from .operations import add, div
from dataclasses import dataclass


@dataclass
class Scenario:
    input: list[int]
    expected: float


class TestOperations(TestCase):
    def test_add(self) -> None:
        scenarios: list[Scenario] = [
            Scenario([10, 30], 40),
            Scenario([10, 0], 10),
        ]

        for scenario in scenarios:
            got = add(*scenario.input)
            self.assertEqual(got, scenario.expected)

    def test_mul(self) -> None:
        scenarios: list[Scenario] = [
            Scenario([100, 10], 10.0),
            Scenario([10, 4], 2.5),
        ]

        for scenario in scenarios:
            got = div(*scenario.input)
            self.assertEqual(got, scenario.expected)

    # test if error is thrown
    def test_mul_throws(self) -> None:
        with self.assertRaises(Exception):
            div(100, 0)

Running the tests

$ python -m unittest ./src/**/*_test.py

Assertions

Type	Meaning
assertEqual(a, b)	a == b
assertNotEqual(a, b)	a != b
assertTrue(x)	bool(x) is True
assertFalse(x)	bool(x) is False
assertIs(a, b)	a is b
assertIsNot(a, b)	a is not b
assertIsNone(x)	x is None
assertIsNotNone(x)	x is not None
assertIn(a, b)	a in b
assertNotIn(a, b)	a not in b
assertIsInstance(a, b)	isinstance(a, b)
assertNotIsInstance(a, b)	not isinstance(a, b)