Standard Library

Testing

Tests make expected behavior executable and repeatable.

A test starts with behavior small enough to name. The function can be ordinary code; the test supplies a representative input and expected result.

Source

def add(left, right):
    return left + right

print(add(2, 3))

Output

unittest.TestCase groups test methods. setUp runs before each test method to build per-test fixtures, assertEqual checks values, and assertRaises asserts that a block raises the expected exception type.

Source

import unittest


def divide(left, right):
    if right == 0:
        raise ZeroDivisionError("denominator is zero")
    return left / right


class AddTests(unittest.TestCase):
    def setUp(self):
        self.zero = 0

    def test_adds_numbers(self):
        self.assertEqual(add(self.zero + 2, 3), 5)

    def test_adds_empty_strings(self):
        self.assertEqual(add("", "py"), "py")

    def test_divide_by_zero_raises(self):
        with self.assertRaises(ZeroDivisionError):
            divide(1, 0)

print([name for name in dir(AddTests) if name.startswith("test_")])

Output

['test_adds_empty_strings', 'test_adds_numbers', 'test_divide_by_zero_raises']

A runner executes the suite and records whether every assertion passed. Capturing the runner stream keeps this page's output deterministic.

Source

import io

loader = unittest.defaultTestLoader
suite = loader.loadTestsFromTestCase(AddTests)
stream = io.StringIO()
runner = unittest.TextTestRunner(stream=stream, verbosity=0)
result = runner.run(suite)
print(result.testsRun)
print(result.wasSuccessful())

Output

3
True

Notes

Test method names should describe behavior, not implementation details.
A good unit test is deterministic and independent of test order.
Use broader integration tests when the behavior depends on several components working together.

Run the complete example

Example code

Expected output

3
True

Execution time appears here after you run the example.

Testing

See also

Run the complete example

Example code

Expected output