Over the course of writing Python in the past year, I’ve learned a bit about writing effective unit-tests. Here is a collection of some of the common use-cases I’ve come across while writing tests in Python.

Unit Testing

unit test basics
what do you test?


mocking methods of a class
mocking out imported libraries
mocking objects

Mock Behavior

returning multiple values
asserting multiple method calls
mock exceptions


Unit Testing

When you’re writing features and new interesting code, the last thing on your mind is writing tests for it. When you’re writing fresh code, the logic seems simple and not worth testing at all. It’s only weeks or months later when you revisit it and make changes that you realize you have no idea what is what. Keeping at least a surface level of test cases allows for you to make changes without worrying about all of the business logic in your applications.

Unit Test Basics

At the core, a unit test is just validation of the behavior of your code. You want to know that your classes and your methods are doing what you expect. When I was first learning how to design tests, breaking them down really helped to design useful ones.

  • if - this section of your unit test sets the stage for the behavior you’re testing. For example, let’s say we want to test that user creation doesn’t create new users with the same name. The if portion of this test would set the stage by creating a user with a given test username to check against later.

  • when - this portion of your unit test actually runs the behavior that you want to test. In the above example, it’s the equivalent of making the actual method call to create a new user with the username you know will fail.

  • then - finally, you want verify that the code called above actually works. In our case, we should see that an exception gets thrown or a 400 Bad Request gets returned.

If you keep this in mind while writing your unit tests, you should end up with sensible tests that verify useful behavior.


The Python standard library has a built-in unit-test module that you can use to structure your tests. The unittest.TestCase class can be inherited to represent a suite of tests that relate to each other. For example, if we want to test the behavior of a class that handles notifications, we would create a subclass of TestCase.

class NotificationTests(unittest.TestCase):

    def setUp(self):
        # set up method that gets called before every test

    def test_notification(self):
        # a test
        self.assertEqual(True, True)

You can add different edge cases and test different outcomes inside a single suite of tests. TestCase also comes with methods like assertListEqual and others that can help make writing tests easier.

What Do You Test?

When writing unit tests, it can be easy to go a little overboard and start testing every function in your code. While you can do this, all it ultimately does is make testing very brittle. During refactoring, your unit tests constantly require changing to keep them up-to-date and the value can get lost. Write unit-tests that test the API’s and interfaces between code and that verify crucial business logic. You don’t need to write tests to verify that you are using the range method properly, for example.


Larger codebases have lots of moving parts and complex behaviors that need to be tested, but not necessarily all at once. I could try to explain this in words, but looking at a practical example may be easier.

def send_notifications(self, message: str):
    total_notifications = 0
    for user in all_users:
        if not user.is_valid():

        self.notifier.send_message(user, message)
        self.db.track_notification(user, message)
        total_notifications += 1

    return total_notifications

In the above example, let’s say we want to test that the logic around validating users and notification counting works as expected. We don’t really want to test the send_message method of notifier, we want to test that independently elsewhere. This is where mocks come in handy. They allow you to test that certain chunks of code are called/referenced without actually calling that piece of code.

In our unit test for the method, all we care about is that send_message got called a certain number of times. Whatever that method does is out of this current test’s scope. The following line also makes an external call to a database. Mocking also allows for you take advantage of design patterns such as dependency injection so that you can test modularly too. Let’s look at a few common mocking scenarios that I’ve run into consistently.

Mocking Methods of a Class

While testing methods of a class, you might come across times where you want to mock out the other methods of a class. We can use the patch method to mock portions of a class without messing with others so you can still use them. Here’s some example code:

class HeartBeater(object):

    def __init__(self, service_name: str, event_name: str, db_cursor)
        self.service_name = service_name
        self.event_name = event_name
        self.active = True

    def update_timeout_flag(self, new_flag: bool):
        # some logic here that makes a database query

    def check_status(self, current_time: int):
        time_difference = current_time - self.last_received
        is_timed_out = time_difference > self.timeout

        if not self.timed_out and is_timed_out:

            return (False, "some error message",)

        elif self.timed_out and is_timed_out:
            return (False, "still timed out",)
            return (True, "some happy path",)

While the class might not be the most sensible, let’s say we want to test the check_status method. Particularly we want to see that we hit the first conditional block where update_timeout_flag is called. Here is how to use patch to turn that call into a mock that effectively does nothing.

import unittest

from mock import patch
from heartbeater import HeartBeater

class HeartBeaterTest(unittest.TestCase):
    def setUp(self):
        # the if portion of our test
        self.hb = HeartBeater('service_test', 'test_event', db_cursor)

    @patch.object(HeartBeater, 'update_timeout_flag')
    def check_status(self, update_timeout_mock):
        # the when portion of our test
        result = self.hb.check_status(141444444)
        # the then portion of our test

As you can see, the patch.object decorator adds the update_timeout_mock variable to our unit test. This is the mock object that now represents the update_timeout_flag method of the class. Whenever we call that method directly or indirectly, the mock will record how it was used so that you can make assertions later.

Mocking Imported Libraries

The next case I occasionally run into deals with mocking out imported libraries. The case I’ll go over here involves mocking methods of the sys package.

# filename: archiver/archive.py

def archive(day, datasource):
    # some logic here..

    sys.stdout.write("some debug message here")

    if not_valid:

If we want to mock out both of these calls, we can use the patch decorator to accomplish what we want. A simple test case for this code would look something like this:

# filename: tests/test_archive.py

# TestCase boilerplate here..

def test_archiver(self, stdout_mock, exit_mock):

    result = archive('03-21-2017', 23)

    stdout_mock.write.assert_called_once_with("some debug message here")

In the unit test, you see that we create 2 distinct patches for both of the calls we want to mock. First we mock out the exit method as well as the stdout module of sys. The unit test being patched must have both in the argument list so we can work with the mocks we’ve created. Mocks come in with some handle calls that help us to verify that the mocks are being used correctly: sys.exit() shouldn’t be called and stdout.write should be written with a debug message.

The final key point to note here is the path of the patch string for sys. You’ll notice that the patch is not on @patch("sys.exit"), this is intentional. Because you want to mock the import call from that module, you must specify the path of the module importing and making the call, archiver.archive.sys.exit. If you don’t do this, your code won’t fail, but the usage of sys in your code won’t get mocked as expected.

This happens for all sorts of imports and its very important, especially when using libraries like boto3 that run configuration code on import.

Mocking Objects

When using database cursors and other configurable objects, I often use dependency injection patterns to make my code easier to test. I end up with methods and classes that require a database cursor.

class HeartBeater(object):

    def __init__(self, db_cursor, metric_registry):
        self.db_cursor = db_cursor
        self.metric_registry = metric_registry

    def get_last_hb(self):
        query = "select * from heartbeater order by last_updated desc limit 1"

        result = self.db_cursor.fetchone()
        return result

To test that code, we can create instances of MagicMock objects to inject upon creation of the HeartBeater object. Let’s look at an example.

class HeartBeaterTest(object):

    def setUp(self):
        self.mock_cursor = MagicMock()
        self.mock_metrics = MagicMock()
        self.test_hb = HeartBeater(self.mock_cursor, self.mock_metrics)

    def test_get_last_hb(self):
        actual_result = self.test_hb.get_last_hb()

        self.mock_cursor.assert_called_with("query string here")

By mocking the objects that we inject into the class instance, all of our subsequent use of that class and its methods can be easily tested. Notice the use of the setUp method also saves us time and allows for us to use the same mock objects throughout our test suite.

Mock Behavior

Now that we’ve learned how to properly mock objects, it’s time to learn how to return expected values and make assertions. Let’s look at 3 different scenarios you might run into: 1) mocks should return multiple values on subsequent calls, 2) asserting a mock was called multiple times, and 3) mocking/asserting an exception was made.

Returning Multiple Values

In your code, you might have a single mock object called multiple times. Each time it is called, it could be expected to return different values. Fortunately, MagicMock allows us to handle this by taking advantage of the side_effect method.

Let’s say we have a method that is supposed to return an integer, and the code we are testing calls it 3 times. You can use the side_effect property as follows to return different values on successive calls.

mock_object = MagicMock()

mock_object.method_to_mock.side_effect = [

You can use this method to mock whatever you’d like to return, from arrays to more complicated objects. One more thing to note is that MagicMock allows for you to mock any chained attributes or methods from it. Let’s say that I have a line of code that looks like this:


# if the heartbeater object is mocked, that means all subsequent calls on it
# are mocked as well

mock_heartbeater.check_status.side_effect = [ True, False, True]
mock_heartbeater.get_latest_hb.side_effect = [ HeartBeater(1), HeartBeater(2) ]

As you can see, the hierarchy underneath the mock objects can also be tested.

Asserting Multiple Calls

The mock library comes with a standard method assert_called_once_with that makes it easy to ensure sure that a mock is called once with parameters of your choice. There is also a lesser known but even more useful method assert_has_calls that tests exactly what it implies. The documentation is pretty useful so I won’t go into too much detail, but here is some example usage.

from unittest.mock import call, MagicMock

mock_object = MagicMock()

expected_calls = [

mock_object.assert_has_calls(expected_calls, any_order=False)

When any_order is True, the assertion does not care about the order in which these calls are made, only that they all exist and were run.

Mocking Exceptions

The final pattern I run into deals with handling exceptions in your code. I often want to test throwing an exception from a mocked object to see how the surrounding code is able to handle it.

The side_effect attribute again comes in handy here. If you would like for a method to throw an exception when called, you set side_effect to be equal to the exception that you’d like.

heartbeater.check_status() # we want this to throw an exception

mock_heartbeater.check_status.side_effect = Exception('boom')

Whenever check_status is called on this mock instance, the exception specified will get thrown.

On the other side of things, the unittest library also makes it very easy to assert that exceptions are thrown. assertRaises makes it simple and encapsulates the code that is throwing the exception.

class TestSomething(unittest.TestCase):

    def test_behavior(self):
        with self.assertRaises(CustomException) as cm:

        self.assertEqual(cm.exception.error_code, 3)

You even can store the exception if you’d like to test attributes of the exception on top of verifying that it was raised.


If you are new to unit-testing with Python, hopefully you have some ideas of how you can get started. The unittest and mock libraries built in to Python3 are very powerful and allow for you to test many different kinds of behavior.

Along the way you are likely to run into some creative scenarios and usages of these libraries, but the basics here will cover many of the common issues and questions you might have. Happy testing!