Ana Balica - 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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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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✓ Setup ✓ Test ✓ Verify state ✓ Teardown ✓ Setup ✓ Setup expectations ✓ Test ✓ Verify expectations ✓ Verify state ✓ Teardown Stubs Mocks

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

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

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Mock the object where it’s used,   not where it came from

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Chapter two

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Good mocks

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with patch.dict('os.environ', {'ANDROID_ARGUMENT': ''}): pf = Platform() self.assertTrue(pf == ‘android') os.environ System calls

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@mock.patch('sys.stdout', new_callable=six.StringIO) def test_print_live_refs_empty(self, stdout): trackref.print_live_refs() self.assertEqual(stdout.getvalue(), 'Live References\n\n\n') sys.stdout Streams

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request.urlopen @patch('django.utils.six.moves.urllib.request.urlopen') def test_oembed_photo_request(self, urlopen): urlopen.return_value = self.dummy_response result = wagtail_oembed("http://www.youtube.com/watch/") self.assertEqual(result['type'], 'photo') Networking

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@patch.object(DataLoader, '_get_file_contents') def test_parse_json_from_file(self, mock_def): mock_def.return_value = ('{"a": 1, "b": 2, "c": 3}', True) output = self._loader.load_from_file('dummy_json.txt') self.assertEqual(output, dict(a=1, b=2, c=3)) json.loads IO operations

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@mock.patch('time.sleep') def test_500_retry(self, sleep_mock): self.set_http_response(status_code=500) # Create a bucket, a key and a file with self.assertRaises(BotoServerError): k.send_file(fail_file) Clocks, time, timezones time.sleep

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with mock.patch('random.random', return_value=0.0): with self.assertChanges(get_timeline_size, before=10, after=5): backend.add(timeline, next(self.records)) random.random Unpredictable results

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

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

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Chapter three

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mocks Bad

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with 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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with 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() Problems?

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¯\_(ϑ)_/¯

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Solution 1 with 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 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 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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Problems? with 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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Tests pass? SHIP IT!

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Maybe   it’s incomplete?

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No content

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

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Unit tests Integration tests Mocks

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Only mock types   that you own

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Building on  Third-Party  Code

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Adapter  layer 3rd party API Application  objects

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Adapter  layer 3rd party API Application  objects Test this cluster Mock this