Python's Built-in unittest Framework

Python comes pre-packaged with a complete, enterprise-grade test runner and assertion library called unittest (based on standard xUnit patterns).

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1. Structure of a unittest Test File

To write a test suite:

1. Import unittest.
2. Import the function you want to test.
3. Create a class that inherits from unittest.TestCase.
4. Create test methods whose names must start with test_.

# calculator.py (Code to test)
def add(a, b):
    return a + b

def divide(a, b):
    if b == 0:
        raise ZeroDivisionError("Cannot divide by zero!")
    return a / b

# test_calculator.py (The Test File)
import unittest
from calculator import add, divide

class TestCalculator(unittest.TestCase):
    
    def test_add_positive_numbers(self):
        result = add(10, 5)
        self.assertEqual(result, 15)

    def test_add_negative_numbers(self):
        result = add(-3, -7)
        self.assertEqual(result, -10)

    def test_divide_valid(self):
        result = divide(10, 2)
        self.assertEqual(result, 5.0)

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

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2. Running Tests from the Terminal

# Run a specific test file
python test_calculator.py

# Run all test files automatically (Test Discovery)
python -m unittest discover

Understanding the Terminal Output

...
----------------------------------------------------------------------
Ran 3 tests in 0.002s

OK

• Each . (dot) represents a passing test.
• An F means an assertion Failed.
• An E means an unhandled Exception crashed your code.

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Quick Summary

Assertion Method	Checks	Example
self.assertEqual(a, b)	a == b	self.assertEqual(add(2, 3), 5)
self.assertNotEqual(a, b)	a != b	self.assertNotEqual(result, 0)
self.assertTrue(x)	bool(x) is True	self.assertTrue(is_logged_in)
self.assertFalse(x)	bool(x) is False	self.assertFalse(is_locked)
self.assertIsNone(x)	x is None	self.assertIsNone(user_found)
self.assertIn(item, list)	item in list	self.assertIn("Python", skills)
self.assertAlmostEqual(a, b)	Floating point match	self.assertAlmostEqual(0.1+0.2, 0.3, places=5)

• Method Naming: Test method names must start with test_ for automatic test discovery.

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What's Next?

Let's learn how to manage database setup, API teardown fixtures, and test exceptions using Writing Test Cases & Test Fixtures (setUp/tearDown)!