[DjangoCon Europe 2016] To mock or not to mock, that is the question

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T o    or not to    that is the question MO CK MO CK

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@anabalica a treatise narrated by    from Potato of Londontowne. ANA BALICA

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Thou   shalt   write   tests. “ ” William Shakespeare

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Chapter one

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Mocks simulate  the looks and behaviour   of real objects

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REAL fig.1 fig.2 MOCK

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mocks != stubs

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unittest.mock # Python 3.3 mock # Python 2.x

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Mock()

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Mock >>> from mock import Mock >>> m = Mock() >>> m.foo = 1 >>> m.foo 1 >>> m.bar

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Mock() MagicMock()

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__lt__ __gt__ __len__ __iter__ __bool__ __str__ __int__ __hash__ __exit__ __sizeof__

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Mock() MagicMock() patch()

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Patching from mock import patch with patch('rainbow.Pony') as MockPony: MockPony.return_value = 42

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Patching 'rainbow.Pony' # creatures.py class Pony: pass # rainbow.py from creatures import Pony pony = Pony()

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Chapter two

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Good mocks

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System calls with mock.patch.dict("foo.os.environ", {"LOCAL_ENV": "production"}): conf = Configuration() self.assertEqual(conf.env, "production") os.environ

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Streams @mock.patch('sys.stdout', new=StringIO) def test_silly_logger(self, mock_stdout): logger.print_refs() self.assertEqual(mock_stdout.getvalue(), 'Some refs') sys.stdout

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HTTP requests /responses requests.get with mock.patch("foo.requests.get") as mock_get: mock_get.return_value = mock.Mock(status_code=200) status = is_it_down(“http://djangocon.eu") self.assertFalse(status)

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IO operations m = mock.mock_open(read_data="DjangoCon \o/“) with mock.patch("foo.open", m, create=True): with open("bar.txt") as h: result = h.read() self.assertEqual(result, "DjangoCon \o/") open

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Clocks, time, timezones from django.utils import timezone tz = pytz.timezone('Asia/Tokyo') dt = datetime.datetime(2016, 1, 1, tzinfo=tz) with mock.patch.object(timezone, 'now', return_value=dt): print timezone.now() timezone.now

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Unpredictable results with mock.patch('foo.random.random', return_value=0.42): result = throw_dice() self.assertEqual(result, 0.42) random.random

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✓ System calls ✓ Streams ✓ HTTP requests ✓ IO operations ✓ Clocks, time, timezones ✓ Unpredictable results

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Why we like them ✓ Save time ✓ Make impossible possible ✓ Exclude external dependencies

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Chapter three

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Bad mocks

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with mock.patch.object(Pony, 'save_base') as mock_save: form = PonyForm(instance=pony, data=data) self.assertTrue(form.is_valid()) saved_pony = form.save() self.assertTrue(saved_pony.age, 3) mock_save.assert_called_once() Problems?

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Problems? with mock.patch.object(Pony, 'save_base') as mock_save: form = PonyForm(instance=pony, data=data) self.assertTrue(form.is_valid()) saved_pony = form.save() self.assertEqual(saved_pony.age, 3) mock_save.assert_called_once()

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¯\_(ツ)_/¯

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Solution 1 with mock.patch.object(Pony, 'save_base') as mock_save: form = PonyForm(instance=pony, data=data) self.assertTrue(form.is_valid()) saved_pony = form.save() self.assertEqual(saved_pony.age, 3) mock_save.assert_called_once_with()

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Solution 2 with mock.patch.object(Pony, 'save_base') as mock_save: form = PonyForm(instance=pony, data=data) self.assertTrue(form.is_valid()) saved_pony = form.save() self.assertEqual(saved_pony.age, 3) self.assertEqual(mock_save.call_count, 1)

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Failure with mock.patch.object(Pony, 'save_base') as mock_save: form = PonyForm(instance=pony, data=data) self.assertTrue(form.is_valid()) saved_pony = form.save() self.assertEqual(saved_pony.age, 3) self.assertEqual(mock_save.sandwich_count, 1)

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Solution 3 Test Driven Development

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Tests pass? SHIP IT!

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Maybe   it’s incomplete?

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No content

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c = Client() response = c.post('/pony/', {'age': 1}) self.assertEqual(response.status_code, 201) Functional tests