Python Unit Testing

Unit testing is a critical portion of any significant software project. Althougth adding unit tests increases the size of your project’s code base, well written unit tests let us maintain confidence in our code base.

Well designed code should work well as stand alone or mostly stand alone software components. This is true of both procedural code and OOP. Unit tests test these components to make sure they continue to work as expected. It helps development because if a software components breaks an expected interface or starts behaving in an expected fashion, we will know about the issue prior to building or deploying our application.

Many developers (including myself) prefer to know about bugs before users see them. Writing good units are one of many tools that help us catch bugs before they make it out into production code. This post will walk us through Python’s unit testing framework.

Example Test Class and Unit Test

Let’s start by creating a class that we are going to unit test. We are going to make a Greeter class that takes a Gender enumeration and a Greeter class.

from enum import Enum

class Gender(Enum):
    MALE = "m"
    FEMALE = "f"

class Greeter:
    def __init__(self, name, gender): = name
        self.gender = gender

    def greet(self):
        if self.gender == Gender.MALE:
            return 'Hello Mr. {}'.format(
        elif self.gender == Gender.FEMALE:
            return 'Hello Ms. {}'.format(

What we are expecting the Greeter.greet method to do is print a greeting that contains either Mr or Ms depending on the gender. Let’s make a test that makes sure we are getting the correct output.

import unittest

class TestGreeter(unittest.TestCase):
    def test_greet(self):
        # Create a Greeter Object to test
        greeter_male = Greeter('Jonny', Gender.MALE)

        # Now check it is working properly
        self.assertTrue('Mr' in greeter_male.greet(), 'Expected Mr')

        # Now check female
        greeter_female = Greeter('Jane', Gender.FEMALE)
        self.assertTrue('Ms.' in greeter_female.greet(), 'Expected Ms')

if __name__ == '__main__':
    # This invokes all unit tests

We get the following output in the console. It’s pretty boring.

Ran 1 test in 0.002s


Catching Bugs

Our first test is what we see if we have a working class and unit test. It’s very boring and we want boring. In many software projects, we tend to change things as we add new features, fix bugs, or improve code. If our changes break things in our code base, we want our unit tests to tell us about the issue.

Let’s make a small change in our Greeter class

class Greeter:
    def __init__(self, name, gender): = name
        self.gender = gender

    def greet(self):
        if self.gender == Gender.MALE:
            return 'Hello Mr. {}'.format(
        elif self.gender == Gender.FEMALE:
            # Changed Ms. to Mrs.
            return 'Hello Mrs. {}'.format(

Now let’s run our test and see what happens

Traceback (most recent call last):
  File "/usr/local/Cellar/python3/3.6.1/Frameworks/Python.framework/Versions/3.6/lib/python3.6/unittest/", line 59, in testPartExecutor
  File "/usr/local/Cellar/python3/3.6.1/Frameworks/Python.framework/Versions/3.6/lib/python3.6/unittest/", line 601, in run
  File "/Users/stonesoup/PycharmProjects/stonesoupprogramming/", line 35, in test_greet
    self.assertTrue('Ms.' in greeter_female.greet(), 'Expected Ms')
  File "/usr/local/Cellar/python3/3.6.1/Frameworks/Python.framework/Versions/3.6/lib/python3.6/unittest/", line 678, in assertTrue
    raise self.failureException(msg)
AssertionError: False is not true : Expected Ms

Ran 1 test in 0.013s

FAILED (failures=1)

If you are looking closely, we changed Ms. to Mrs. in our greeting. Given how small the change, it’s really easy for our human eyes to overlook the change and anyone can image how easy it would be for this bug to make it into production. Since our unit test is well written, we know about this bug right away!

If we did want our message to print Mrs rather than Ms, we need to update our unit test. That’s a good thing because it makes us think about how changes to our code impact the code base in general. Unit tests are so helpful that many developers have even adopted to “Test Driven Development” programming discipline.

You can learn more about Python’s unit testing framework at here.


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