Mocking | Rustam Zokirov

Test double: dummy, fake, stub, mock
Mocks
Stubs
Difference between stub and mock
References

Test double: dummy, fake, stub, mock

Test doubles = faking before testing. Use mocking or stubbing when your code uses external dependencies like system calls, or accessing a database. To create independent tests and run more quickly.

Fake is used when prod code is not ready. It is imitating. Similar to stub.

Stub is incoming, for getting input.

Mock is out-coming, making request to other dependencies to change their state. Mock is saving what calls were made.

Mocks

Моки помогают имитировать и изучать исходящие (out coming) взаимодействия. То есть вызовы, совершаемые тестируемой системой (SUT) к ее зависимостям для изменения их состояния.
Mocking means creating a fake version of an external or internal service that can stand in for the real one. Helping your tests run more quickly and more reliably.
It allows you to replace parts of your system under test with mock objects.
Mocks are objects that register calls they receive. In test assertion we can verify on Mocks that all expected actions were performed.
Python has Mock and MagicMock classes to create mocks. MagicMock as an extended version of Mock. MagicMock has the implementation of magic methods.

# Mock returns Mock instance when calling object itself, or accessing one of its attributes or call method.
mock = mock.Mock()
mock.foo = 'bar'
assert mock.foo == 'bar'

mock.configure_mock(bar='baz')
assert mock.bar == 'baz'

mock = mock.Mock(foo='bar')
assert mock.foo == 'bar'

mock = mock.Mock()
mock.return_value = 42
assert mock() == 42

# Returning different values on each call using side_effect
mock.side_effect = ['foo', 'bar', 'baz']
assert mock() == 'foo'
assert mock() == 'bar'
assert mock() == 'baz'
try:
    mock()
except StopIteration:
    assert True
else:
    assert False

# Raising an exception when calling the Mock
mock.side_effect = RuntimeError('Boom') # OR Mock(side_effect=KeyError('foo'))
try:
    mock()
except RuntimeError:
    assert True
else:
    assert False

The Mock objects record all interactions with them and you can then inspect these interactions.

mock = mock.Mock()

mock(1, foo='bar')      #=> <Mock name='mock()' id='4522261528'>
mock.foo()              #=> <Mock name='mock.foo()' id='4524858840'>
mock(2, bar='foo')      #=> <Mock name='mock()' id='4522261528'>
mock.foo(bar=123)       #=> <Mock name='mock.foo()' id='4524858840'>

mock.call_args_list     #=> [call(1, foo='bar'), call(2, bar='foo')]
mock.method_calls       #=> [call.foo(), call.foo(bar=123)]

mock.reset_mock()
mock.call_args_list     #=> []
mock.method_calls       #=> []

dir(mock.Mock())

'assert_called_once', 'assert_called_once_with', 'assert_called_with', 
'assert_any_call', 'assert_called', 'assert_has_calls','assert_not_called', 

'call_args_list', 'mock_calls', 'method_calls',
'return_value', 'side_effect', 'reset_mock', 'configure_mock'
'attach_mock', 'call_args',  'call_count', 'called', 'mock_add_spec'

# Attaching mock attribute to an existing object
class ProductionClass: pass

mock = mock.Mock(name='foo', return_value='bar')
thing = ProductionClass()
thing.method = mock

thing.method(1, 2, k='v')         #=> 'bar'
mock.call_args_list               #=> [call(1, 2, k='v')]
mock.method_calls                 #=> []

The patch()function looks up an object in a given module and replaces it with another object. Replaced with MagicMock.

‣

Nice description

import requests
from unittest.mock import patch


def my_func():
    response_data = requests.get("https://jsonplaceholder.typicode.com/todos/1").text
    response_data_upper = response_data.upper()
    print(response_data_upper)
    return response_data_upper


my_func() # ORIGINAL


@patch(f'{__name__}.requests.get')
def check_my_func(mock_write):
    mock_write.return_value.text = "rustam"
    # Use side_effect if there are multiple similar function with different arguments
    # https://stackoverflow.com/questions/68075655/python-mock-patch-two-functions-that-are-similar
    x = my_func()
    assert x == "RUSTAM"


check_my_func() # RUSTAM
my_func() # ORIGINAL

Stubs

Стабы помогают имитировать входящие (incoming) взаимодействия. То есть вызовы, совершаемые SUT к ее зависимостям для получения входных данных.
Stub is an object that holds predefined data and uses it to answer calls during tests.

Difference between stub and mock

Mock: Например, отправка электронной почты является исходящим (out going) взаимодействием: это взаимодействие приводит к побочному эффекту на SMTP-сервере. Тестовый двойник, имитирующий такое взаимодействие, - это мок.

Stub: Извлечение данных из БД является входящим (incoming) взаимодействием — оно не приводит к побочному эффекту. Соответствующий тестовый двойник является стабом.

References

https://habr.com/ru/post/577424/ mock vs stub vs fake

https://stackoverflow.com/questions/3459287/whats-the-difference-between-a-mock-stub

Watch https://www.youtube.com/watch?v=ww1UsGZV8fQ and https://www.youtube.com/watch?v=Ldlz4V-UCFw and https://www.youtube.com/watch?v=rk-f3B-eMkI