Testing is a critical part of software development. It ensures your code works as expected and prevents future changes from breaking existing functionality. Python provides built-in tools for testing, as well as highly popular third-party frameworks.

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1. unittest module

The unittest module is built into the Python standard library. It is inspired by Java’s JUnit and provides a solid framework for constructing and running tests.

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2. Writing test cases & assert methods

To write a test case, you must create a class that inherits from unittest.TestCase.

Inside this class, any method that starts with the word test_ will be automatically run by the test runner.

import unittest

# The function we want to test
def add(a, b):
    return a + b

# The test suite
class TestMathOperations(unittest.TestCase):

    def test_add_positive_numbers(self):
        # We use assert methods to verify expectations
        self.assertEqual(add(2, 3), 5)

    def test_add_negative_numbers(self):
        self.assertEqual(add(-1, -1), -2)

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

Common Assert Methods

• assertEqual(a, b): Checks if a == b
• assertNotEqual(a, b): Checks if a != b
• assertTrue(x): Checks if bool(x) is True
• assertFalse(x): Checks if bool(x) is False
• assertIsNone(x): Checks if x is None
• assertIn(a, b): Checks if a is in b
• assertRaises(exc, fun, *args, **kwds): Checks if calling the function throws the specified exception.

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3. Running tests

If you added if __name__ == '__main__': unittest.main() to the bottom of your script, you can run the test file directly from the command line:

python test_math.py

Alternatively, you can use the command-line interface of the unittest module to discover and run all tests in a directory automatically:

python -m unittest discover

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4. pytest basics

While unittest is built-in, the most popular testing framework in the Python community is pytest. It requires much less boilerplate code and produces highly readable output.

You must install it first:

pip install pytest

With pytest, you don’t need to create classes or use self.assertEqual. You just write normal functions and use the standard Python assert keyword.

# test_sample.py
def add(a, b):
    return a + b

def test_add_positive():
    assert add(2, 3) == 5

def test_add_negative():
    assert add(-1, -1) == -2

To run the tests, simply open the terminal and run:

pytest

pytest will automatically find any files starting with test_ or ending with _test.py, run all functions inside them starting with test_, and report the results.