Introduction to unittest (Part 2): Assertions, test discovery, and running tests

Part 1 showed TestCase scaffolding. You now need richer assertions, lifecycle hooks, and predictable discovery so suites scale past a handful of files.

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📚 Prerequisites

• Basic unittest.TestCase examples.

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🎯 What you'll learn

• Choose specific assertion helpers for clearer failure output.
• Control discovery flags (-k, -f, verbosity).
• Use setUp / tearDown without entangling shared global state.

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Assertion cookbook

Method	When to use
assertEqual(a, b)	Value equality with helpful diffs
assertAlmostEqual	Floating tolerances
assertRaises / assertRaisesRegex	Expected errors
assertIsNone / assertIsNotNone	Optional returns
assertIn / assertCountEqual	Collections membership
assertTrue / assertFalse	Last resort—prefer concrete checks

import unittest

def divide(a: float, b: float) -> float:
    if b == 0:
        raise ValueError("b must be nonzero")
    return a / b

class TestDivide(unittest.TestCase):
    def test_zero_denominator(self):
        with self.assertRaises(ValueError):
            divide(10, 0)

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Discovery tips

python -m unittest discover -s tests -p "test_*.py" -v

-s sets the start directory; -p tweaks the pattern when your organization standardizes naming; -v prints each test method for long CI logs.

Stop on first failure while iterating locally:

python -m unittest discover -f

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Setup and teardown hooks

class TestTmp(unittest.TestCase):
    def setUp(self):
        self.fixture = {"count": 0}

    def tearDown(self):
        # release resources opened in setUp
        self.fixture.clear()

Prefer fresh fixtures per test unless setUpClass’s expensive resource is immutable (read-only databases, compiled models).

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💡 Key takeaways

• Tests should read like specifications; verbose assertion methods double as documentation for future maintainers.

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➡️ Next steps

Switch to pytest’s function style in Introduction to pytest (Part 1).