Write tests with unittest

unittest is part of Python. Test methods start with test_, and assertEqual reports a useful failure when actual and expected values differ.

Worked example

import unittest

def total(values):
    return sum(values)

class TotalTests(unittest.TestCase):
    def test_regular_values(self):
        self.assertEqual(total([20, 25]), 45)
    def test_empty_values(self):
        self.assertEqual(total([]), 0)

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

..
----------------------------------------------------------------------
Ran 2 tests in 0.000s

OK

Why this works

The test loader finds both test_ methods. Save the file as test_total.py and run python test_total.py, or run python -m unittest to discover every test_*.py file in the folder. Each dot is one passing test. A failing assertEqual reports the expected and actual values.

Another case

import unittest
case = unittest.TestCase()
case.assertEqual(sum([1, 2, 3]), 7)

Traceback (most recent call last):
  File "assert_demo.py", line 3, in <module>
    case.assertEqual(sum([1, 2, 3]), 7)
  File ".../unittest/case.py", line ..., in assertEqual
    assertion_func(first, second, msg=msg)
  File ".../unittest/case.py", line ..., in _baseAssertEqual
    raise self.failureException(msg)
AssertionError: 6 != 7

The two case.py entries come from inside unittest itself; their path and line numbers depend on your Python installation. Read the first entry for your own file and the last line for the result.

Your terminal may show the full path to the file instead of just its name. Python 3.11 and newer also add ^ or ~ marker lines under the part of the line that failed, and recent versions colour the output. The last line, with the error type and message, is the same.

Practice

Add a test for total([-5, 10]). What should it expect?

Try another case

You rename test_empty_values to empty_values. How many tests run now?