Introduction to Python’s unittest Library

What is unittest?

unittest is Python’s built-in testing framework.

It provides:

• the TestCase class
• assertion methods (assertEqual, assertTrue, …)
• test discovery
• setup/teardown hooks

Core building blocks

• TestCase: a class that contains test methods
• test method: any method starting with test_
• assertions: checks that validate expected behavior

Minimal example

test_math_unittest.py

import unittest
 
 
def add(a, b):
    return a + b
 
 
class TestAdd(unittest.TestCase):
    def test_add_two_numbers(self):
        self.assertEqual(add(2, 3), 5)
 
 
if __name__ == "__main__":
    unittest.main()

How unittest runs tests

unittest:

• finds test classes derived from unittest.TestCase
• finds methods starting with test_
• runs them and reports failures/errors

When to use unittest

• you want zero external dependencies
• you’re working in legacy codebases already using unittest

(Modern projects often prefer pytest; we cover it in Phase 4.)

Visualize it

Every test method in a TestCase runs between a fresh setUp() and tearDown() call, so each test starts and ends in a known state.

diagram unittest Test Lifecycle mermaid

  flowchart TD
A["setUp()"] --> B["Test method (assertions)"]
B --> C["tearDown()"]
C --> D["Repeat for each test method"]
D --> A

setUp() and tearDown() wrap around every individual test method.

🧪 Try It Yourself

Exercise 1 – Write a unittest TestCase

pch.exerciseTag pch.exerciseTitle

Exercise 2 – assertRaises

pch.exerciseTag pch.exerciseTitle

Exercise 3 – setUp and tearDown

pch.exerciseTag pch.exerciseTitle