unittest: TestCase, Assertions & Fixtures

unittest is Python's built-in testing framework, part of the standard library -- no installation required. It follows the xUnit style pioneered by JUnit: tests grouped into classes, with setup/teardown lifecycle hooks and a rich family of assert* methods.

Writing a TestCase

import unittest

class TestMathUtils(unittest.TestCase):
    def test_addition(self):
        self.assertEqual(2 + 2, 4)

    def test_division_by_zero_raises(self):
        with self.assertRaises(ZeroDivisionError):
            1 / 0

    def test_string_contains(self):
        self.assertIn('py', 'python')

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

Only methods whose names start with test are picked up as tests. assertEqual, assertIn, assertRaises, etc. produce detailed failure output automatically -- meaningfully better than a bare assert statement, which gives no context on failure.

setUp & tearDown: Per-Test Fixtures

import tempfile
import shutil

class TestFileProcessor(unittest.TestCase):
    def setUp(self):
        # Runs before EACH test method -- guarantees fresh, isolated state
        self.temp_dir = tempfile.mkdtemp()

    def tearDown(self):
        # Runs after EACH test method, even if it raised an exception --
        # reliable cleanup regardless of pass/fail
        shutil.rmtree(self.temp_dir)

    def test_writes_output_file(self):
        result_path = process_file('input.txt', self.temp_dir)
        self.assertTrue(result_path.endswith('.out'))

setUpClass: Shared, Expensive Setup

class TestUserRepository(unittest.TestCase):
    @classmethod
    def setUpClass(cls):
        # Runs ONCE for the whole class -- for setup genuinely safe to
        # share across tests, like an expensive DB connection
        cls.db = connect_to_test_db()

    @classmethod
    def tearDownClass(cls):
        cls.db.close()

    def test_user_lookup(self):
        user = self.db.find_user('alice')
        self.assertIsInstance(user, dict)

Common Assertion Methods

• assertEqual(a, b) / assertNotEqual(a, b) -- value equality, with a detailed diff on failure.

• assertTrue(x) / assertFalse(x) -- truthiness checks.

• assertIn(item, container) / assertNotIn(item, container) -- membership.

• assertIsInstance(obj, cls) -- type checking.

• assertAlmostEqual(a, b) -- floating-point comparison within a tolerance, since 0.1 + 0.2 == 0.3 is False due to floating-point representation.

• assertRaises(ExceptionType) -- used as a context manager to assert an exception is raised.