Lesson 28: Unit Testing in Python

🎯 Lesson Objective

To understand how to test Python code automatically using the unittest module, ensuring functions and classes behave as expected.

🧩 1. What Is Unit Testing?

Unit Testing involves testing individual units (functions, methods, or classes) to verify correctness.

Benefits:

Catch bugs early
Ensure reliability
Facilitate safe refactoring

⚙️ 2. Importing the `unittest` Module

import unittest

🧱 3. Structure of a Unit Test

Import unittest and the module to test
Create a class inheriting from unittest.TestCase
Define test methods starting with test_
Use assertions to check results
Run tests using unittest.main()

Example 1 — Testing a Simple Function

Code to test (math_ops.py):

def add(a, b):
    return a + b

def multiply(a, b):
    return a * b

Unit Test (test_math_ops.py):

import unittest
from math_ops import add, multiply

class TestMathOps(unittest.TestCase):
    def test_add(self):
        self.assertEqual(add(2, 3), 5)
        self.assertEqual(add(-1, 1), 0)

    def test_multiply(self):
        self.assertEqual(multiply(2, 3), 6)
        self.assertNotEqual(multiply(2, 0), 1)

if __name__ == "__main__":
    unittest.main()

Output:

..
----------------------------------------------------------------------
Ran 2 tests in 0.001s

OK

🔹 4. Common Assertions

Assertion	Purpose
`assertEqual(a, b)`	Check a == b
`assertNotEqual(a, b)`	Check a != b
`assertTrue(x)`	Check x is True
`assertFalse(x)`	Check x is False
`assertIsNone(x)`	Check x is None
`assertIsNotNone(x)`	Check x is not None
`assertRaises(Exception, func, *args)`	Check function raises exception

Example 2 — Testing Exceptions

def divide(a, b):
    return a / b

import unittest

class TestDivide(unittest.TestCase):
    def test_divide_by_zero(self):
        with self.assertRaises(ZeroDivisionError):
            divide(10, 0)

if __name__ == "__main__":
    unittest.main()

Output:

.
----------------------------------------------------------------------
Ran 1 test in 0.000s

OK

Example 3 — Testing Classes

Code to test (employee.py):

class Employee:
    def __init__(self, name, salary):
        self.name = name
        self.salary = salary

    def apply_raise(self, percent):
        self.salary += self.salary * percent

Unit Test (test_employee.py):

import unittest
from employee import Employee

class TestEmployee(unittest.TestCase):
    def setUp(self):
        self.emp = Employee("Alice", 50000)

    def test_apply_raise(self):
        self.emp.apply_raise(0.1)
        self.assertEqual(self.emp.salary, 55000)

if __name__ == "__main__":
    unittest.main()

🔹 5. Running Tests

Command line:

python -m unittest test_math_ops.py

Discover all tests automatically:

python -m unittest discover

🔹 6. Test Fixtures

setUp() → runs before each test
tearDown() → runs after each test

def setUp(self):
    self.file = open("data.txt", "w")

def tearDown(self):
    self.file.close()

🔹 7. Using Mock Objects

Use unittest.mock to replace real objects in tests.

from unittest.mock import patch
import requests

def get_status(url):
    response = requests.get(url)
    return response.status_code

import unittest

class TestAPI(unittest.TestCase):
    @patch('requests.get')
    def test_get_status(self, mock_get):
        mock_get.return_value.status_code = 200
        status = get_status("https://example.com")
        self.assertEqual(status, 200)