This document discusses the proper use of mocks in unit testing. It provides examples of both good and bad uses of mocks. Some key points made include: - Mocks are useful for isolating tests from external dependencies like databases, networks, or unpredictable functions. - Tests using mocks should focus on the code being tested, not the mocks themselves. Asserting properties of mocks directly can lead to brittle tests. - Mock only types that the code being tested owns. External dependencies should be abstracted behind adapter layers and the adapters tested instead of the external code. - While mocks can help write tests, passing tests don't guarantee correct behavior. Integration tests are still needed to validate overall system function

1. To
or not to
that is the question
MO CK
MO CK

2. To
or not to
that is the question
MO CK
MO CK

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

4. Thou
shalt
write
tests.
“ ”
William Shakespeare

5. Chapter one

6. Mocks simulate
the looks and behaviour
of real objects

7. REAL
fig.1 fig.2
MOCK

8. mocks != stubs

9. ✓ Setup
✓ Test
✓ Verify state
✓ Teardown
✓ Setup
✓ Setup expectations
✓ Test
✓ Verify expectations
✓ Verify state
✓ Teardown
Stubs Mocks

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

11. Mock()

12. Mock
>>> from mock import Mock
>>> m = Mock()
>>> m.foo = 1
>>> m.foo
1
>>> m.bar
<Mock name='mock.bar' id='4310136016'>

13. Mock()
MagicMock()

14. __lt__
__gt__
__len__
__iter__
__bool__
__str__
__int__
__hash__
__exit__
__sizeof__

15. Mock()
MagicMock()
patch()

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

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

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

19. Chapter two

20. Good
mocks

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

22. @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 Referencesnnn')
sys.stdout
Streams

23. 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

24. @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

25. @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

26. 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

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

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

29. Chapter three

30. mocks
Bad

31. 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?

32. 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?

33. 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?

34. 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?

35. 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_twice()
Problems?

36. 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.make_me_sandwich()
Problems?

37. ¯_( )_/¯

38. 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()

39. 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)

40. 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)

41. Solution 3
Test Driven
Development

42. 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)

43. Tests
pass?
SHIP IT!

44. Maybe
it’s incomplete?

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

47. Unit tests
Integration
tests
Mocks

48. Only mock types
that you own

49. Building on
Third-Party
Code

50. Adapter
layer
3rd party
API
Application
objects

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

52. Conclusions

53. Mocks
can be
dangerous

54. Passing faulty tests give a
false sense of security

55. The end