A concise guide to writing unit tests, mocking, and measuring test coverage in Python using unittest library.

Unit testing ensures code reliability. This guide covers:
✅ Writing unit tests with unittest
✅ Mocking dependencies
✅ Measuring test coverage

📂 Project Structure

Organize test cases effectively:

project_root/
│── src/
│   ├── utils.py  # Utility functions
│   ├── operations.py  # Business logic
│── tests/
│   ├── test_operations.py  # Test cases

✅ Writing Tests

📌 src/operations.py 

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

📌 tests/test_operations.py 

import unittest
from src.operations import add

class TestMathOperations(unittest.TestCase):
    def test_add(self):
        self.assertEqual(add(5, -12), -7)

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

Run tests:

python -m unittest discover -s tests

🔄 Setup & Teardown 

import unittest
from io import StringIO
from src.utils import say_hello
from unittest.mock import patch

class TestGreetings(unittest.TestCase):
    def setUp(self):
        print("SETUP")

    def tearDown(self):
        print("TEARDOWN")

    def test_say_hello(self):
        with patch('sys.stdout', new=StringIO()) as fake_out:
            say_hello()
            self.assertEqual(fake_out.getvalue().strip(), "Hello World")

🔍 Mocking with unittest.mock 

# src/data_fetcher.py
def get_status():
    return "Live"

def fetch_data():
    return {"status": get_status()}
# tests/test_data_fetcher.py
import unittest
from unittest.mock import patch
from src.data_fetcher import fetch_data

class TestDataFetcher(unittest.TestCase):
    @patch("src.data_fetcher.get_status")
    def test_fetch_data(self, mock_get_status):
        mock_get_status.return_value = "Mocked Status"
        self.assertDictEqual(fetch_data(), {"status": "Mocked Status"})

Run tests:

python -m unittest tests.test_data_fetcher

📈 Measuring Test Coverage 

pip install coverage
coverage run -m unittest discover -s tests
coverage html  # Generates an HTML report

View htmlcov/index.html for insights.

🚀 Conclusion

unittest helps write reliable tests. Use mocking to isolate dependencies and coverage to track completeness.

Start writing better tests today! 🧪✨