Managing mocks

The how, why and when of mocking in Python

Helen Sherwood-Taylor

EuroPython

21st July 2016

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
not just for unittest

>>> from unittest import mock


# Or, for Python <3.3
$ pip install mock

>>> import mock

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

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

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

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

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)])

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

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
```
assert*
```
```
assret*
```
disable it with unsafe=True

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)

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)

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

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

patch

Get access to anything

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

creates a MagicMock

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

patch context manager

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

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()

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

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

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.

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'

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(mock_date):
    mock_date.today.return_value = date(2016, 3, 19)

    assert birthday.its_my_birthday()

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()

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

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 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'

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

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'

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

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

Thank you!

Twitter: @helenst

Github: @helenst

http://helen.st/

https://github.com/helenst/managing_mocks

Managing Mocks

By Helen Sherwood-Taylor

Managing Mocks

For EuroPython 2016

2,733

Managing mocks

The how, why and when of mocking in Python

What is a mock?

Why?

unittest.mock

Mock objects

Callable Mocks

Side effects

Multiple side effects

What was called?

Beware versions

assert safety

Matching call args

Comparisons

Call inspection

How to use all this

patch

patch decorator

patch context manager

patch: manual control

Where to patch

Where to patch: 2

Where to patch: the answer

patch.object

Mock the time

Is it my birthday?

freezegun

Mock the filesystem

Iterate a mock file

Mock a property

How to mock a property

Example: delay loop

When to mock?

What about the rest?

Further reading

Thank you!

Managing Mocks

More from Helen Sherwood-Taylor