Entertaining testing with pytest

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Entertaining testing with pytest Roman Imankulov / @rdotpy / 21 Apr 2017

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Why developers don’t like writing tests?

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Testing in Python is a religion • Original sin • Absolution through pain and suffering • Mystical experience

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Original sin ● Original sin — dynamic typing and duck typing ● As a result, a natural inclination of a Python developer to create little and stupid mistakes exposed as runtime errors

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Absolution through pain and suffering Boilerplate Code class TestSequenceFunctions(unittest.TestCase): def setUp(self): ... def tearDown(self): ... def testFoo(self): ...

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Absolution through pain and suffering Verbose asserts self.assertEqual(foo, 1, 'foo is not equal to one')

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Mystical experience Django testing setups & teardowns

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Are there any alternatives?

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pytest

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file: test_something.py def test_lowercase(): name = "OPOPython" assert name.lower() == "opopython"

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pytest is not a yet another xUnit framework!

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pytest fixtures That’s what makes the difference

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pytest fixtures Naive approach. How I’d do it myself file: fixtures.py def get_user(): return User(name='Roman', age=33, ...) file: test_user.py def test_user(): user = get_user() assert user.name == 'Roman'

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pytest fixtures Pytest approach file: conftest.py @pytest.fixture def user(): return User(name='Roman', age=33, ...) file: test_user.py def test_user(user): assert user.name == 'Roman'

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Fixture dependencies

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@pytest.fixture def user(): return User(name='Roman', age=33, ...) @pytest.fixture def task(user): return Task(user=user, content='...') def test_task(task): assert task.user.name == 'Roman'

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Fixture dependencies. Patching object @pytest.fixture def premium(user) user.set_premium() def test_premium(user, premium): assert user.is_premum()

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yield_fixture 2 in 1, setup and teardown

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@pytest.yield_fixture def user(): obj = User(name='Roman', age=30, ...) yield obj obj.delete()

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Fixture scopes • function scope • module scope • session scope

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Session fixture. Local cache @pytest.yield_fixture(scope='session', autouse=True) def local_cache(): old_settings = settings.CACHES settings.CACHES = {'default': {…}} yield settings.CACHES = old_settings

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Function fixture. Database transaction rollback @pytest.yield_fixture def tx(): db().start_transaction() yield db().rollback() def test_user(user, tx, project, task): # project & task will be removed automatically

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Session fixture. Clean redis @pytest.yield_fixture(scope='session') def redis_server(): proc = subp.Popen(['redis-server', '--port', 7777], ... ) yield proc proc.terminate() @pytest.fixture def rc(redis_server): client = redis.StrictRedis('redis://127.0.0.1:7777') client.flushall() return client

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fixtures parametrization

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Function returning a function @pytest.fixture def set_lang(user): def func(lang_code): user.set_lang(lang_code) return func def test_languages(user, set_lang): set_lang('pt') ...

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Crazy things

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Fixtures in a separate thread http://bit.ly/test_pool @pytest.fixture(scope='session') def item_gen(): gen = Generator(lambda: .) gen.start() return gen @pytest.yield_fixture def item(item_gen, item_rel): item = item_gen.get() yield item item_rel.put(item) @pytest.fixture(scope='session') def item_rel(): rel = Releaser(lambda o: ...) rel.start() return rel

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More use cases for fixtures • warnings: turn MySQL warnings to errors • mock: initialize mockup objects • freezegun: time management • selenium: run a web driver

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What else @pytest.mark.parametrize("input,expected", [ ("3+5", 8), ("2+4", 6), ]) def test_eval(input, expected): assert eval(input) == expected

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What else def pytest_addoption(parser): parser.addoption("--clean-mysql", action="store_true", default=False) @pytest.fixture(scope='session', autouse=True) def clean_mysql(request): if not request.config.getoption(‘--clean-mysql'): return # clean MySQL tables heres

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What else • pytest-sugar: beautiful output • pytest-django: integration with Django • pytest-xdist: parallel and distributed testing

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What else • tox: testing against different versions of Python • detox: the same, but in parallel [tox] envlist = py27,py35,py36 [testenv] deps=pytest commands=py.test

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Thank you! Questions? Roman Imankulov / @rdotpy