Mocks and Monkeypatching in Python
This tutorial will help you understand why mocking is important, and show you how to mock in Python with Mock and Pytest monkeypatch.
Post originally published on http://krzysztofzuraw.com/. Republished with author's permission.
In this post I will look into the essential part of testing — mocks.
Why bother mocking?
Some of the parts of our application may have dependencies for other libraries or objects. To isolate the behaviour of our parts, we need to substitute external dependencies. Here comes the mocking. We mock an external API to check certain behaviours, such as proper return values, that we previously defined.
Let’s say we have a module called
def square(value): return value ** 2 def cube(value): return value ** 3 def main(value): return square(value) + cube(value)
Then let’s see how these functions are mocked using the
try: import mock except ImportError: from unittest import mock import unittest from function import square, main class TestNotMockedFunction(unittest.TestCase): @mock.patch('__main__.square', return_value=1) def test_function(self, mocked_square): # because you need to patch in exact place where function that has to be mocked is called self.assertEquals(square(5), 1) @mock.patch('function.square') @mock.patch('function.cube') def test_main_function(self, mocked_square, mocked_cube): # underling function are mocks so calling main(5) will return mock mocked_square.return_value = 1 mocked_cube.return_value = 0 self.assertEquals(main(5), 1) mocked_square.assert_called_once_with(5) mocked_cube.assert_called_once_with(5) if __name__ == '__main__': unittest.main()
What is happening here? Lines 1-4 are for making this code compatible between Python 2 and 3. In Python 3, mock is part of
the standard library, whereas in Python 2 you need to install it by
pip install mock.
In line 13, I patched the
square function. You have to remember to patch it in the same place you use it. For instance,
square(5) in the test itself so I need to patch it in
__main__. This is the case if I’m running this by
python tests/test_function.py. If I’m using pytest for that, I need to
patch it as
In lines 18-19, I patch the
cube functions in their module because they are used in the
The last two asserts come from the mock library, and are there to make sure that mock was called with proper values.
The same can be accomplished using
mokeypatching for py.test:
from function import square, main def test_function(monkeypatch): monkeypatch.setattr(“test_function_pytest.square”, lambda x: 1) assert square(5) == 1 def test_main_function(monkeypatch): monkeypatch.setattr(‘function.square’, lambda x: 1) monkeypatch.setattr(‘function.cube’, lambda x: 0) assert main(5) == 1
As you can see, I’m using
monkeypatch.setattr for setting up a return value for given functions. I still need to
monkeypatch it in proper places —
I have a module called
import math class Square(object): def __init__(radius): self.radius = radius def calculate_area(self): return math.sqrt(self.radius) * math.pi
and mocks using standard lib:
try: import mock except ImportError: from unittest import mock import unittest from square import Square class TestClass(unittest.TestCase): @mock.patch('__main__.Square') # depends in witch from is run def test_mocking_instance(self, mocked_instance): mocked_instance = mocked_instance.return_value mocked_instance.calculate_area.return_value = 1 sq = Square(100) self.assertEquals(sq.calculate_area(), 1) def test_mocking_classes(self): sq = Square sq.calculate_area = mock.MagicMock(return_value=1) self.assertEquals(sq.calculate_area(), 1) @mock.patch.object(Square, 'calculate_area') def test_mocking_class_methods(self, mocked_method): mocked_method.return_value = 20 self.assertEquals(Square.calculate_area(), 20) if __name__ == ‘__main__’: unittest.main()
At line 13, I patch the class
Lines 15 and 16 present a mocking instance.
mocked_instance is a mock object which returns another mock by default,
and to these
mock.calculate_area I add
return_value 1. In line 23, I’m using
MagicMock, which is a normal mock class, except in that it also retrieves magic methods from the given object. Lastly,
patch.object to mock the method in the
The same using pytest:
try: from mock import MagicMock except ImportError: from unittest.mock import MagicMock from square import Square def test_mocking_class_methods(monkeypatch): monkeypatch.setattr('test_class_pytest.Square.calculate_area', lambda: 1) assert Square.calculate_area() == 1 def test_mocking_classes(monkeypatch): monkeypatch.setattr('test_class_pytest.Square', MagicMock(Square)) sq = Square sq.calculate_area.return_value = 1 assert sq.calculate_area() == 1
The issue here is with
test_mocking_class_methods, which works well in Python 3, but not in Python 2.
All examples can be found in this repo.
If you have any questions and comments, feel free to leave them in the section below.