Python Tutorial / Write Automated Tests with unittest and Boundary Cases

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Automated tests repeatedly check defined results or exceptions for selected inputs. Test the behavior users require, including boundaries and failures, so a later implementation change can reveal a regression.

Define the rules before testing

The total function accepts a list of nonnegative integers, returns zero for an empty list and rejects bool and strings. Start with a small calculation independent of the clock, network and user files to keep its tests reproducible.

unittest belongs to the standard library. Name test files and test methods with the test_ prefix. Unlike a person inspecting printed output, assertions turn a mismatch into a machine-visible failed test.

For the relevant APIs and platform requirements, see Write Automated Tests with unittest and Boundary Cases documentation.

Save the complete function and test files

In a fresh folder, save expense_math.py:

def total(amounts):
    if any(type(x) is not int or x < 0 for x in amounts):
        raise ValueError("invalid amount")
    return sum(amounts)

Save test_expense_math.py alongside it:

import unittest
from expense_math import total

class TotalTests(unittest.TestCase):
    def test_regular(self):
        self.assertEqual(total([3500, 8000, 2500]), 14000)

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

    def test_invalid(self):
        for values in [[-1], [True], ["8000"]]:
            with self.subTest(values=values):
                with self.assertRaises(ValueError):
                    total(values)

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

Run from that folder:

python -m unittest -v

Expect three tests and a final OK. The invalid-input test uses subTest to distinguish three cases. Timing and exact layout vary by environment.

Confirm the tests detect a faulty implementation

Temporarily return sum(amounts) + 1 and verify that regular and empty-input tests fail.
Restore the result and check for OK.
Remove input validation, inspect failures for invalid cases and then restore it.

A deliberate mutation checks that a test actually protects its intended rule. Do not leave the intentionally broken implementation as the final example.

Keep unit tests distinct from integration checks

A passing total test cannot establish that a CSV header or database connection is correct. Integration tests connecting files or databases need independent temporary paths and controlled data rather than production records.

Record the interpreter version, command and results when reporting verification. State which inputs were tested rather than extending the claim to installation screens or unrelated environments. The CSV project offers concrete file-level integration checks.

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