Python course / Applied

Testing

Write tests that fail when the code is wrong, and make hard-to-test code testable. A test that passes whatever the code does is worse than no test, because it buys false confidence.

assertions
boundaries
unittest
setUp
subTest
doctest
mocks and patching
async tests

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a case table and a failure

import unittest


def portion(total, people):
    if people <= 0:
        raise ValueError("people must be positive")
    return total / people


class PortionTests(unittest.TestCase):
    def test_shares(self):
        for total, people, each in [(10, 4, 2.5), (9, 3, 3.0)]:
            with self.subTest(total=total, people=people):
                self.assertEqual(portion(total, people), each)

    def test_refuses_nobody(self):
        with self.assertRaises(ValueError):
            portion(10, 0)


unittest.main(argv=["tests"], exit=False, verbosity=2)

test_refuses_nobody (__main__.PortionTests.test_refuses_nobody) ... ok
test_shares (__main__.PortionTests.test_shares) ... ok

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

OK

What you will learn

Write test functions that call code and assert what it should give, strong enough to fail when the code is wrong.
Pick test cases at the boundaries and for the errors, where mistakes live, and test that code raises by hand.
Write tests as a unittest.TestCase with assertEqual and assertRaises, and run them with unittest.main.
Make code that reads the clock testable by passing the clock in, and test it with fakes.
Replace something a function looks up for the length of a test with unittest.mock.patch, patching the name where it is used.
Write tests that check what code does for its callers, so they survive a rewrite of how it does it.

The lessons in this module

What a test is
Choosing the cases
unittest
Fresh objects for every test: setUp
Tables of cases: subTest
Examples that check themselves: doctest
Passing in what varies
Mock objects
Patching
Testing input and output
Testing with files
Testing async code
Testing properties
Test behaviour, not insides
Tests first
A test for every bug
Capstone: a test suite for an inventory
Module review: a table-driven test runner

Ready when you are

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