Managing mocks

The how, why and when of mocking in Python

Helen Sherwood-Taylor

PyCon UK

19th September 2015

What is a mock?

A fake object that replaces a real object

Control and isolate the test's environment

Why?

Deterministic tests

Control the test inputs

Faster test suites

unittest.mock

also known as the mock library
written by Michael Foord
Added to Python in 3.3
Rolling backport for earlier Pythons


# Python 2.x, <3.3
$ pip install mock

>>> import mock

# Python 3.3+
>>> from unittest import mock

Mock objects

>>> from unittest.mock import Mock
>>> mock_talk = Mock(conference='PyCon UK')
>>> mock_talk.conference
'PyCon UK'

>>> 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'

>>> mock_talk.speaker.get_twitter_status()
<Mock name='mock.speaker.get_twitter_status()' id='139910592221312'>
>>> mock_talk.speaker.get_twitter_status.return_value = 'Eating cheese'
>>> mock_talk.speaker.get_twitter_status()
'Eating cheese'

Side effects

An exception

>>> mock_func = Mock()
>>> mock_func.side_effect = Exception
>>> mock_func()
Traceback (most recent call last):
...
Exception

A function

>>> mock_func.side_effect = lambda x: x*2
>>> mock_func(2)
4

Multiple side effects

A list of return values and exceptions

>>> mock_func.side_effect = [1, ValueError, 2]
>>> mock_func()
1
>>> mock_func()
Traceback (most recent call last):
...
ValueError
>>> mock_func()
2
>>> mock_func()
Traceback (most recent call last):
...
StopIteration

What was called?

```
assert_called_with(*args,**kwargs)
```

assert_called_once_with(*args, **kwargs)

```
assert_any_call(*args, **kwargs)
```
```
assert_has_calls(*args, **kwargs)
```

>>> 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)])

New in Python 3.5 - assert_not_called

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)

Matchers

Use matcher 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)

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

How to use all this

Action / Assertion mechanics

Create a mock
Set up inputs / environment
Run the code under test
Assert expectations met

Direct creation

Mock another method on object under test

def test_greeting(self):
    user = User()
    user.is_birthday = Mock(return_value=True)

    greeting = user.greet()

    self.assertEqual(greeting, 'happy birthday')

patch

Get access to anything

>>> patch('requests.get')
<unittest.mock._patch object at 0x7f7a2af03588>

Decorator
Context manager
Manual control

creates a MagicMock

patch decorator

import github3

def get_followers(username):
    api_client = github3.GitHub()
    return api_client.user(username).followers

class GitHubTest(unittest.TestCase):
    @patch('github3.GitHub')
    def test_get_followers(self, mockGitHub):
        mockGitHub.return_value.user.return_value.followers = 20
        followers = get_followers('helenst')
	self.assertEqual(followers, 20)

mock is a parameter to the test method

patch context manager

import github3

def get_followers(username):
    api_client = github3.GitHub()
    return api_client.user(username).followers

class GitHubTest(TestCase):

    def test_get_followers(self):
	with patch('github3.GitHub') as mockGitHub:
	    mockGitHub.return_value.user.return_value.followers = 20
	    followers = get_followers('helenst')
	    self.assertEqual(followers, 20)

patch: manual control

import github3

def get_followers(username):
    api_client = github3.GitHub()
    return api_client.user(username).followers

class GitHubTest(TestCase):
    def test_get_followers(self):
	patcher = patch('github3.GitHub')
	mockGitHub = patcher.start()
     	mockGitHub.return_value.user.return_value.followers = 20
        followers = get_followers('helenst')
        self.assertEqual(followers, 20)
        patcher.stop()

Where to patch

import github3

def get_followers(username):
    api_client = github3.GitHub()
    return api_client.user(username).followers

import github_utils

class GitHubTest(unittest.TestCase):
    @patch('github3.GitHub')
    def test_get_followers(self, mockGitHub):
        mockGitHub.return_value.user.return_value.followers = 200
        followers = github_utils.get_followers('helenst')
        self.assertEqual(followers, 200)

Where to patch: 2

from github3 import GitHub

def get_followers(username):
    api_client = GitHub()
    return api_client.user(username).followers

import github_utils

class GitHubTest(unittest.TestCase):
    @patch('github3.GitHub')
    def test_get_followers(self, mockGitHub):
        mockGitHub.return_value.user.return_value.followers = 200
        followers = github_utils.get_followers('helenst')
        self.assertEqual(followers, 200)

AssertionError: 14 != 200

Where to patch: the answer

@patch('github_utils.GitHub')

import github_utils

class GitHubTest(unittest.TestCase):
    @patch('github_utils.GitHub')
    def test_get_followers(self, mockGitHub):
        mockGitHub.return_value.user.return_value.followers = 200
        followers = github_utils.get_followers('helenst')
        self.assertEqual(followers, 200)

Patch the imported path

ensure that you patch the name used by the system under test.

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 3rd party APIs

any data you want

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(self, mock_sleep):
    mock_func = Mock(side_effect=[DatabaseError, DatabaseError, 'Yay'])

    result = retry_with_delay(mock_func)

    self.assertEqual(result, 'Yay')
    self.assertEqual(mock_func.call_count, 3)
    mock_sleep.assert_has_calls([
        call(1),
        call(2),
    ])

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.

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(self, mock_date):
    mock_date.today.return_value = date(2015, 3, 19)

    self.assertTrue(birthday.its_my_birthday())

Mock the filesystem

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

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)

Mock a read only 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!'

>>> person = Person()
>>> person.is_birthday = True
Traceback (most recent call last):
...
AttributeError: can't set attribute

How to 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!'

person = Person()
with patch.object(
    Person, 'is_birthday', new_callable=PropertyMock
) as mock_is_birthday:
    mock_is_birthday.return_value = True
    person.greet()

patch.object to the rescue!

What about the rest?

Purity / Design
Speed
Pinpoint failures
Realistic coverage

Harder to read
Harder to write
Sometimes seem meaningless
Doesn't test inter-layer stuff

Should I mock the other layers?

YES

Thank you!

Twitter: @helenst

Github: @helenst

http://helen.st/

Managing Mocks

By Helen Sherwood-Taylor

Managing Mocks

For PyCon UK 2015

3,973

Managing mocks

The how, why and when of mocking in Python

What is a mock?

Why?

unittest.mock

Mock objects

Side effects

Multiple side effects

What was called?

Matching call args

Matchers

Call inspection

How to use all this

Direct creation

patch

patch decorator

patch context manager

patch: manual control

Where to patch

Where to patch: 2

Where to patch: the answer

When to mock?

Example: delay loop

Mock the time

Is it my birthday?

Mock the filesystem

Iterate a mock file

Mock a read only property

How to mock a property

What about the rest?

Thank you!

Managing Mocks

More from Helen Sherwood-Taylor