From Europython 2016 Mocking is a valuable technique for writing tests but mocking effectively is often a stumbling block for many developers and can raise questions about its overall value as a technique. The audience will have some familiarity with unit testing and may have tried mocking before, but some introduction will be provided for those who haven’t. We will look at some features and techniques of Python’s unittest.mock library and cover some useful tips and common scenarios, so this will be useful to those who have some experience mocking but would like to do so more effectively. Video: https://www.youtube.com/watch?v=Ahnw72diels

1. Managing mocks
The how, why and when of mocking in Python
Helen Sherwood-Taylor
EuroPython
21st July 2016

2. What is a mock?
A fake object that replaces a real object
Control and isolate the test's environment

3. Why?
Deterministic tests
Control the test inputs
Faster test suites

4. unittest.mock
>>> from unittest import mock
# Or, for Python <3.3
$ pip install mock
>>> import mock
also known as the mock library
written by Michael Foord
Added to Python in 3.3
Rolling backport for earlier Pythons
not just for unittest

5. Mock objects
>>> from unittest.mock import Mock
>>> mock_talk = Mock(conference='EuroPython')
>>> mock_talk.conference
'EuroPython'
>>> mock_talk.speaker
<Mock name='mock.speaker' id='139910592223552'>
>>> mock_talk.speaker.name
<Mock name='mock.speaker.name' id='139910592222208'>
>>> mock_talk.speaker.name = 'Helen'
>>> mock_talk.speaker.name
'Helen'
Properties of Mocks

6. Callable Mocks
>>> mock_talk.speaker.get_twitter_status()
<Mock name='mock.speaker.get_twitter_status()'>
>>> mock_talk.speaker.
get_twitter_status.return_value = 'Eating pintxos'
>>> mock_talk.speaker.get_twitter_status()
'Eating pintxos'
return_value

7. Side effects
An exception
>>> mock_func = Mock()
>>> mock_func.side_effect = ValueError
>>> mock_func()
Traceback (most recent call last):
...
ValueError
A function
>>> mock_func.side_effect = lambda x: x*2
>>> mock_func(2)
4

8. Multiple side effects
A list of return values and exceptions
>>> mock_func.side_effect = [1, ValueError, 2]
>>> mock_func()
Traceback (most recent call last):
...
ValueError
>>> mock_func()
Traceback (most recent call last):
...
StopIteration
>>> mock_func()
1
>>> mock_func()
2

9. What was called?
assert_called_with(*args, **kwargs)
assert_called_once_with(*args, **kwargs)
assert_any_call(*args, **kwargs)
assert_has_calls((call(*args, **kwargs), ...))
assert_not_called()
>>> from unittest.mock import Mock, call
>>> mock_func = Mock()
>>> mock_func(1)
>>> mock_func(2)
>>> mock_func.assert_called_with(2)
>>> mock_func.assert_has_calls([call(1), call(2)])

10. Beware versions
assert_not_called
>>> mock_func = Mock()
>>> mock_func()
>>> mock_func.assert_not_called()
<Mock name='mock.assert_not_called()' id='139758629541536'>
python 3.5
>>> mock_func = Mock()
>>> mock_func()
>>> mock_func.assert_not_called()
...
AssertionError: Expected 'mock' to not have been called. Called 1 times.
python 3.4

11. assert safety
python 3.5
>>> from unittest.mock import Mock, call
>>> mock_func = Mock()
>>> mock_func.assert_called()
...
AttributeError: assert_called
Detects non existent assertion calls
disable it with unsafe=True
assert*
assret*

12. Matching call args
mock.ANY doesn't care
>>> from unittest.mock import Mock, ANY
>>> mock_func = Mock()
>>> mock_func(25)
>>> mock_func.assert_called_with(ANY)

13. Comparisons
Use comparison objects for more control
class MultipleMatcher:
def __init__(self, factor):
self.factor = factor
def __eq__(self, other):
return other % self.factor == 0
def __repr__(self):
return 'Multiple of {}'.format(self.factor)
>>> mock_func = Mock()
>>> mock_func(25)
>>> mock_func.assert_called_with(MultipleMatcher(5))
>>> mock_func.assert_called_with(MultipleMatcher(4))
...
AssertionError: Expected call: mock_func(Multiple of 4)
Actual call: mock_func(25)

14. Call inspection
Lower level inspection of calls
called - was it called?
call_count - how many times?
call_args - args/kwargs of last call
call_args_list - args/kwargs of all calls

15. How to use all this
1. Create / insert a mock
2. Set up inputs / environment
3. Run the code under test
4. Assert expectations met

16. patch
Get access to anything
>>> from unittest.mock import patch
>>> patch('requests.get')
<unittest.mock._patch object at 0x7f7a2af03588>
creates a MagicMock

17. patch decorator
import requests
def get_followers(username):
response = requests.get(
'https://api.github.com/users/%s' % username
)
return response.json()['followers']
@patch('requests.get')
def test_get_followers(mock_get):
mock_get.return_value.json.return_value = {'followers': 100}
assert get_followers('somebody') == 100
mock is a parameter to the test method
>>> print(get_followers('helenst'))
25

18. patch context manager
with patch('requests.get') as mock_get:
mock_get.return_value.json.return_value = {'followers': 100}
assert get_followers('somebody') == 100

19. patch: manual control
patcher = patch('requests.get')
mock_get = patcher.start()
mock_get.return_value.json.return_value = {'followers': 100}
assert get_followers('somebody') == 100
patcher.stop()

20. Where to patch
# github_utils.py
import requests
def get_followers(username):
response = requests.get(
'https://api.github.com/users/%s' % username
)
return response.json()['followers']
import github_utils
with patch('requests.get') as mock_get:
mock_get.return_value.json.return_value = {'followers': 100}
# This will succeed.
assert github_utils.get_followers('somebody') == 100

21. Where to patch: 2
# file: github_utils2
from requests import get
def get_followers(username):
response = get(
'https://api.github.com/users/%s' % username
)
return response.json()['followers']
import github_utils2
with patch('requests.get') as mock_get:
mock_get.return_value.json.return_value = {'followers': 100}
assert github_utils2.get_followers('somebody') == 100
AssertionError: 25 != 100

22. Where to patch: the answer
patch('github_utils2.get')
import github_utils2
with patch('github_utils2.get') as mock_get:
mock_get.return_value.json.return_value = {'followers': 100}
# This will succeed.
assert github_utils2.get_followers('somebody') == 100
Patch the imported path
“ ensure that you patch the name used by the system under test.

23. patch.object
Attach mocks to an object
class User:
def is_birthday(self):
# ...
pass
def greet(self):
if self.is_birthday():
return 'happy birthday'
else:
return 'hello'
user = User()
with patch.object(user, 'is_birthday', return_value=True):
# user.is_birthday is a MagicMock that returns True
# Check the outcome
assert user.greet() == 'happy birthday'

24. Mock the time
>>> patch("datetime.date.today").start()
...
TypeError: can't set attributes of built-in/extension type 'datetime.date'
date is written in C so you can't mock its attributes.
>>> patch('datetime.date').start()
<MagicMock name='date' id='140222796874696'>
But you can mock the whole thing.

25. Is it my birthday?
from datetime import date
def its_my_birthday():
today = date.today()
return today.month == 3 and today.day == 19
import birthday
@patch('birthday.date')
def test_birthday_3(mock_date):
mock_date.today.return_value = date(2016, 3, 19)
assert birthday.its_my_birthday()

26. freezegun
$ pip install freezegun
from freezegun import freeze_time
import birthday
@freeze_time('2016-03-19')
def test_birthday_4():
assert birthday.its_my_birthday()

27. Mock the filesystem
from unittest.mock import mock_open
open_mock = mock_open(
read_data='look at all my file contents'
)
with patch('__main__.open', open_mock, create=True):
with open('myfile') as file:
assert file.read() == 'look at all my file contents'
mock_open will help you out here.
But you can't iterate

28. Iterate a mock file
with patch('__main__.open', mock_open(), create=True) as open_mock:
mock_file = open_mock.return_value.__enter__.return_value
mock_file.__iter__.return_value = ([
'line one', 'line two'
])
with open('myfile') as file:
assert list(file) == ['line one', 'line two']
with open('myfile') as myfile:
for line in myfile:
print(line)

29. Mock a property
class Person:
@property
def is_birthday(self):
today = date.today()
return self.dob.month == today.month and self.dob.day == today.day
def greet(self):
return 'Happy birthday!' if self.is_birthday else 'Good morning!'
# Patching the object doesn't work.
>>> with patch.object(person, 'is_birthday', return_value=True):
... assert person.greet() == 'Happy birthday!'
...
AttributeError: <__main__.Person object at 0x7f9875ea73c8>
does not have the attribute 'is_birthday'

30. How to mock a property
with patch.object(
Person,
'is_birthday',
new_callable=PropertyMock,
return_value=True
):
assert person.greet() == 'Happy birthday!'
use new_callable
patch the class

31. Example: delay loop
def retry_with_delay(func):
delay = 1
while True:
try:
return func()
except DatabaseError:
time.sleep(delay)
delay *= 2
@patch('time.sleep')
def test_third_time_lucky(mock_sleep):
mock_func = Mock(side_effect=[DatabaseError, DatabaseError, 'Yay'])
result = retry_with_delay(mock_func)
assert mock_func.call_count == 3
mock_sleep.assert_has_calls([
call(1),
call(2),
])
assert result == 'Yay'

32. When to mock?
Current time
say happy birthday to a user
Simulated failure
(what happens when the disk is full?)
Slowness
e.g. time.sleep
Randomness
Remote APIs
any data you want

33. What about the rest?
Purity / Design
Speed
Pinpoint failures
Realistic coverage
Harder to read
Harder to write
Meaningless?
Doesn't test inter-layer stuff
Should I mock the other layers?
YES NO

34. Further reading
Book: http://www.obeythetestinggoat.com/
Talk: Gary Bernhardt - Fast Test, Slow Test
https://docs.python.org/3/library/unittest.mock.html

35. Thank you!
Twitter: @helenst
Github: @helenst
http://helen.st/
https://github.com/helenst/managing_mocks