unittest: Your First Real Suite

Assertions in a script stop at the first failure. A test suite runs every case, reports all failures, and becomes your safety net for refactoring. unittest ships with Python:

import unittest

class TestInvoice(unittest.TestCase):
    def setUp(self):                       # fresh fixture per test
        self.invoice = Invoice(["tea", "cup"])

    def test_total_sums_items(self):
        self.assertEqual(self.invoice.total(), 7)

    def test_empty_invoice_is_zero(self):
        self.assertEqual(Invoice([]).total(), 0)

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

What you gain over bare asserts:

• Independent tests: setUp runs fresh before each method — no shared mutable state, no order dependence.
• Rich failure output: expected vs actual, per test.
• A runner: python -m unittest discovers and runs everything; the platform's sandbox runs your tests the same way.

Test naming is communication: test_total_sums_items says what behavior is expected. When a test fails months from now, the name is the first hint.