Python — Simple examples for mock module

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Mock bird

This article will bring you some simple examples for learning the mock module in unit tests.

System environment

  • OS: CentOs 8
  • Python version: 3.6.8


These days I was assigned a task to make the unit tests completed. To make the tests simple, I learned to utilize the mock module in Python. I had heard about this module for a while but didn’t have a chance to understand it clearly. Now I regret that I didn’t learn it early. (Yeah, it’s my fault.)

What can the mock module offer to you?

It’s a very useful tool to cooperate with the unittest module. Assumed that you knew how to write unit tests. We write unit tests for each module to make sure they work perfectly. But sometimes some modules rely on others. We don’t want to get over the scope of the module we want to test. Or it’s difficult to trigger the special case to complete out tests. Mock can help you a lot. Download this code.

Starting from the unit tests

Here we have a class Demo to be tested.


import datetimeclass ValueError(Exception):
class SpecialError(Exception):
class Bias():

def get_bias():
return 5

class Demo:
def __init__(self, num1, num2):
self._num1 = num1
self._num2 = num2
def num1(self):
return self._num1

def num2(self):
return self._num2
def _special_number(self):
''' This might be a function using third-party methods '''
return 1
def sum(self):
n = self._special_number()
if n < 0:
raise ValueError
return self._num1 + self._num2 + n

def sum_bias(self):
return self._num1 + self._num2 + Bias.get_bias()
def append_datetime(self, string):
return str( + string

This is a unit test file.


import unittest
from unittest.mock import Mock
from src.demo import Demo
class TestDemo(unittest.TestCase): def test_basic(self):
demo = Demo(1, 2)
self.assertEqual(1, demo.num1)
self.assertEqual(2, demo.num2)
self.assertEqual(1, demo._special_number())
self.assertEqual(4, demo.sum())
self.assertEqual(8, demo.sum_bias())

Try it

$ python3 -m unittest tests/

Add mock into tests

You can see we only have one test. If _special_number() gives a negative, sum() throws a exception. We can try this case without modifying the source code.

from unittest.mock import Mock, MagicMock, patch
from src.demo import Demo, ValueError, SpecialError

class TestDemo(unittest.TestCase):

def test_raise_error(self):
demo = Demo(1, 2)
demo._special_number = MagicMock(return_value=-1)
with self.assertRaises(ValueError):

The MagicMock object can replace the returning value of _special_number as -1. So that we can test the error cases easily. The below code snippet have the same effect, it just uses with to do it.

class TestDemo(unittest.TestCase):

def test_raise_error_patch(self):
with patch.object(Demo, '_special_number', return_value=-1) as mocked_mathod:
demo = Demo(1, 2)
with self.assertRaises(ValueError):

We can also make it raise an exception when the function is called. The assigned side_effect will be thrown out so it can be caught.

class TestDemo(unittest.TestCase):

def test_raise_error_side_effect(self):
demo = Demo(1, 2)
demo._special_number = MagicMock(side_effect=SpecialError)
with self.assertRaises(SpecialError):

The real world is always complicated. None of the modules works alone. However, I want the tests to be simple. We can mock those third-party libraries to generate our testing cases.

class TestDemo(unittest.TestCase):

def test_mock_sum_bias(self):
demo = Demo(1, 2)
self.assertEqual(8, demo.sum_bias())

def test_sum_bias(self, mocked_bias):
mocked_bias.get_bias.return_value = 7
demo = Demo(1, 2)
self.assertEqual(10, demo.sum_bias())

The patch decorator with arguments src.demo.Bias would mock the Bias class until this function is ended. Therefore we can modify the returning value as we want.

In the Blow code snippet, we mock the datatime module. It shows the power of this module in writing tests. The gives a string “This is a mocked method” rather than the string of current time because we replace the function result.

class TestDemo(unittest.TestCase):

def test_datetime(self):
demo = Demo(1, 2)
self.assertTrue('abc' in demo.append_datetime('abc'))

def test_mock_datetime(self, mocked_datetime): = 'This is a mocked method'
demo = Demo(1, 2)
self.assertTrue('mocked' in demo.append_datetime('abc'))

Try to imagine that you have a module that needs to use the requests module to download files from the network. But you don’t want to waste time downloading real files. In this case, you could mock the requests module to simplify your testing.

Simple is better.

If this article helps you, please give m a clap. Thanks!

From Taiwan, a beautiful island. Learning English and sharing code experience.

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