Testing Python Code with unittest and pytest

Automated tests are code that verifies your other code behaves correctly, run repeatedly and automatically rather than checked by hand every time. Python ships with a built-in testing framework called unittest, and the third-party pytest package has become the de facto standard for most modern Python projects because of its simpler syntax and richer features.

Writing tests catches regressions early — when a change to one part of a codebase accidentally breaks another part — and serves as executable documentation of how a function is actually supposed to behave.

Why it matters

Code without tests can only be checked by running it by hand, and every change risks breaking something that used to work. Automated tests check it in seconds, every time. Writing tests is a standard part of professional development: most teams will not accept a change without them, and many interviews include a testing question.

assert Statements — The Foundation of Testing

At its core, a test checks that some condition is true, and Python's built-in assert statement does exactly that: assert condition, "optional message" does nothing if the condition is true, and raises an AssertionError (with the optional message) if it's false. Both unittest and pytest build on this same fundamental idea, just with more structure and better failure reporting around it.

Writing Tests with unittest

unittest is class-based: you subclass unittest.TestCase and write methods whose names start with test_. Instead of plain assert, TestCase provides methods like self.assertEqual(a, b), self.assertTrue(x), and self.assertRaises(ExceptionType), which produce clearer failure messages than a bare assert would. Running python -m unittest from a project's root automatically discovers and runs any file matching the pattern test_*.py containing TestCase subclasses.

Writing Tests with pytest

pytest lets you write tests as plain functions (no class or special base class required) using ordinary assert statements — it rewrites assertions internally to produce detailed, readable failure output showing exactly which values didn't match. Like unittest, it automatically discovers test files (again, files and functions named test_* or *_test by default) and runs them when you invoke the pytest command in a project directory, without needing to import or register each test file manually.

Example

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

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


# file: test_calculator_unittest.py
import unittest
from calculator import add, divide

class TestCalculator(unittest.TestCase):
    def test_add(self):
        self.assertEqual(add(2, 3), 5)

    def test_divide_by_zero_raises(self):
        with self.assertRaises(ValueError):
            divide(10, 0)

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


# file: test_calculator_pytest.py
from calculator import add, divide
import pytest

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

def test_divide_by_zero_raises():
    with pytest.raises(ValueError):
        divide(10, 0)

# Run with: python -m unittest        (discovers test_calculator_unittest.py)
# Run with: pytest                    (discovers test_calculator_pytest.py)

----------------------------------------------------------------------
Ran 2 tests in 0.001s

OK
================= 2 passed in 0.01s =================

Exercise

Write slugify, which turns a title into a lower-case string with hyphens in place of spaces, and two test functions for it in the pytest style, using assert. With pytest installed, the tests would be run with the pytest command; here they are called directly, and a message is printed if both pass.

Expected output: 2 passed

def slugify(title):
    # TODO: strip surrounding spaces, convert to lower case, replace spaces with hyphens
    pass


def test_replaces_spaces():
    # TODO: assert that "Hello World" becomes "hello-world"
    pass


def test_strips_outer_spaces():
    # TODO: assert that "  Python  " becomes "python"
    pass


test_replaces_spaces()
test_strips_outer_spaces()
print("2 passed")

def slugify(title):
    return title.strip().lower().replace(" ", "-")


def test_replaces_spaces():
    assert slugify("Hello World") == "hello-world"


def test_strips_outer_spaces():
    assert slugify("  Python  ") == "python"


test_replaces_spaces()
test_strips_outer_spaces()
print("2 passed")

Quick quiz

Interview questions