Testing with unittest

In This Chapter

• How to write tests using Python's built-in unittest framework
• How to organize test classes and use setUp / tearDown
• The most common assertion methods and when to use each
• How to run tests from the command line
• When to use unittest vs pytest

What is unittest?

unittest is Python's built-in testing framework, so there is nothing extra to install. It is modeled after Java's JUnit, which is why tests are written as classes that inherit from unittest.TestCase.

import unittest

class TestMath(unittest.TestCase):

    def test_add(self):
        self.assertEqual(1 + 1, 2)

    def test_subtract(self):
        self.assertEqual(5 - 3, 2)

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

Run the file directly:

python test_math.py

Structure

Every test class inherits from unittest.TestCase, and every test method must start with test_.

import unittest

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

class TestAdd(unittest.TestCase):

    def test_positive(self):
        self.assertEqual(add(2, 3), 5)

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

    def test_zero(self):
        self.assertEqual(add(0, 0), 0)

setUp and tearDown

setUp runs before each test. tearDown runs after each test, even if the test fails:

class TestDatabase(unittest.TestCase):

    def setUp(self):
        self.db = connect_database()   # runs before every test

    def tearDown(self):
        self.db.close()                # runs after every test

    def test_insert(self):
        self.db.insert("item")
        self.assertEqual(self.db.count(), 1)

Assertion Methods

unittest uses assertion methods instead of plain assert:

def test_raises(self):
    with self.assertRaises(ValueError):
        int("not a number")

pytest vs unittest

pytest can discover and run unittest-style tests automatically, so the two work well together. Most new projects prefer pytest; unittest still shows up often in older codebases, internal tooling, and standard library code.

Key Questions

Q: What is the difference between pytest and unittest?

unittest is Python's built-in framework, modeled after Java's JUnit — tests are written as methods inside a class that inherits from unittest.TestCase, and assertions use methods like self.assertEqual(). pytest is a third-party library where tests are plain functions using assert. pytest produces more readable output and has more powerful features (fixtures, parametrize). pytest can also run unittest tests, so they're compatible.

Q: What is the purpose of setUp and tearDown in unittest?

setUp runs before each individual test method to prepare the test environment (e.g., create a database connection, initialize objects). tearDown runs after each test to clean up — even if the test failed. They're the unittest equivalent of pytest's fixture setup and teardown with yield.

Q: Why does unittest use self.assertEqual() instead of plain assert?

self.assertEqual() and other TestCase assertion methods provide better failure messages — they show you the actual vs. expected values when a test fails. Plain assert just raises AssertionError with no details. pytest works around this by rewriting assert statements at collection time to produce similarly detailed output.