Lecture 11: Testing

Testing is the process of verifying that your code performs as expected under different conditions. Python's standard library includes the unittest module to facilitate unit testing.

1. Why Test Code?

Prevents regressions: Ensures new changes don't break existing functionality.
Validates assumptions: Confirms the code handles edge cases, bad inputs, and normal cases correctly.
Improves design: Writing tests forces you to design modular, clean functions.

2. Writing a Simple Unit Test

A unit test verifies that a single, specific aspect of a function's behavior is correct.

Let's say you have a function in name_formatter.py:

# name_formatter.py
def get_formatted_name(first, last):
    return f"{first} {last}".title()

You can write a test case for this function by inheriting from unittest.TestCase:

# test_name_formatter.py
import unittest
from name_formatter import get_formatted_name

class NamesTestCase(unittest.TestCase):
    """Tests for 'name_formatter.py'."""
    
    def test_first_last_name(self):
        """Do names like 'Janis Joplin' work?"""
        formatted_name = get_formatted_name('janis', 'joplin')
        self.assertEqual(formatted_name, 'Janis Joplin')

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

Running the Test: Run the test script directly: python test_name_formatter.py.
Test Methods: All methods in a test case starting with test_ will be run automatically.

3. Common Assertion Methods

Assertions verify that a condition evaluates to the expected result.

Assertion Method	Checks
`self.assertEqual(a, b)`	`a == b`
`self.assertNotEqual(a, b)`	`a != b`
`self.assertTrue(x)`	`x is True`
`self.assertFalse(x)`	`x is False`
`self.assertIn(item, list)`	`item in list`
`self.assertNotIn(item, list)`	`item not in list`
`self.assertRaises(Error)`	Code raises `Error`

4. The `setUp()` Method

When testing a class, you often create the same instances and resources in every test. The setUp() method runs before each test method, allowing you to create resources once and access them as instance variables.

class TestAnonymousSurvey(unittest.TestCase):
    def setUp(self):
        """Create survey and responses once for all tests."""
        self.survey = AnonymousSurvey("What is your favorite language?")
        self.responses = ['Python', 'Go', 'JS']

    def test_single_response(self):
        self.survey.store_response(self.responses[0])
        self.assertIn(self.responses[0], self.survey.responses)

🧠 Try It Yourself

Write a function that accepts city and country, returning a formatted string. Write a test case using unittest to test it.
Introduce a bug deliberately in the function (e.g., skip capitalizing the name) and run the test suite to observe the failure.

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