The Benefits of Having One Test Class per Method in Python Testing
When writing tests in Python, it’s common to have multiple test methods in one test class that test different functionalities(methods) of a class. This practice can lead to several issues, such as long and unreadable test classes, difficulty in debugging, and a lack of clarity in what each test is testing.
In this article, we will explore why having one test class per method in Python is a better practice and how it can improve the quality of your tests.
The Traditional Approach
Traditionally, many Python developers write tests in the same class that corresponds to the class being tested. For example, if you have a Django model named BlogPost, you might write a test class named TestBlogPost that contains multiple test methods that test different aspects of the BlogPost class.
While this approach might seem logic at first, it can quickly become problematic. As your test class grows, it can become difficult to read and maintain. This can lead to mistakes, bugs, and even more time spent debugging.
One Test Class Per Method Approach
A better approach to organizing tests in Python is to have one test class per method being tested. In this approach, you would create a separate test class for each method that you want to test. For example, if you have a BlogPost model with a method named publish, you would create a test class named TestBlogPostPublish that contains all the test methods that tests the publish method.
This approach might seem more work but it has several advantages over the traditional approach. First, it makes your tests easier to read and understand. Each test class is focused on a single method, so it’s clear what each test is testing. Second, it makes your tests more modular and easier to maintain. If you need to change the behavior of a method, you only need to change the corresponding test class.
A Realistic Example
Let’s consider a hypothetical User model in a Django application. The User model has three methods that we want to test:
authenticate: A method that takes in a username and password and returns a User object if the username and password match a user in the database, or None if the authentication fails.is_staff: A method that returns True if the user is a staff member, or False if the user is not a staff member.has_perm: A method that takes in a permission string and returns True if the user has that permission, or False if the user does not have that permission.
class TestUser(TestCase):
def test_user_can_authenticate_with_correct_credentials(self):
# Test call the method autenticate
...
def test_user_cannot_authenticate_with_incorrect_credentials(self):
# Test call the method autenticate
...
def test_user_is_staff_returns_true_when_user_is_staff_member(self):
# Test call the method is_staff
...
def test_user_is_staff_returns_false_when_user_not_staff_member(self):
# Test call the method is_staff
...
def test_user_is_staff_returns_true_when_user_has_perm(self):
# Test call the method has_perm
...
def test_user_is_staff_returns_false_when_user_has_no_perm(self):
# Test call the method has_perm
...
Looking first at the above test class, you might think it’s well origanized and feel that there is no necessity of spliting it. Well, what if the authenticate method changes and we want the new case to be tested. Simple enough, let’s add that a new test method and that’s it 😜. But what happens if the developer who’s tackling the task add the test at the bottom of the class.
Here is what it will look like:
class TestUser(TestCase):
def test_user_can_authenticate_with_correct_credentials(self):
# Test call the method autenticate
...
def test_user_cannot_authenticate_with_incorrect_credentials(self):
# Test call the method autenticate
...
def test_user_is_staff_returns_true_when_user_is_staff_member(self):
# Test call the method is_staff
...
def test_user_is_staff_returns_false_when_user_not_staff_member(self):
# Test call the method is_staff
...
def test_user_has_perm_returns_true_when_user_has_perm(self):
# Test call the method has_perm
...
def test_user_has_perm_returns_false_when_user_has_no_perm(self):
# Test call the method has_perm
...
# new added test
def test_new_authenticate_method_case(self):
# Test call the method authenticate
...
You’ll notice that reading the tests for each method of the User model becomes complicated and you’ll lose the ability to know all the tests that exist for each of them.
Now let’s try to split the class. For instance, instead of writing all six tests for the hypothetical User model in a single class TestUser, we would have a separate test class for each method: TestUserAuthentication, TestUserIsStaff, and TestUserHasPerm.
Here is what the code for the three test classes would look like:
class TestUserAuthentication(TestCase):
def test_user_can_authenticate_with_correct_credentials(self):
# Test that user can authenticate with correct credentials
...
def test_user_cannot_authenticate_with_incorrect_credentials(self):
# Test that user cannot authenticate with incorrect credentials
...
class TestUserIsStaff(TestCase):
def test_user_is_staff_returns_true_when_user_is_staff_member(self):
# Test call the method is_staff
...
def test_user_is_staff_returns_false_when_user_not_staff_member(self):
# Test call the method is_staff
...
class TestUserHasPerm(TestCase):
def test_user_has_perm_returns_true_when_user_has_perm(self):
# Test call the method has_perm
...
def test_user_has_perm_returns_false_when_user_has_no_perm(self):
# Test call the method has_perm
...
You can notice that each test class only contains the test methods that test the corresponding method in the User model, making it much easier to navigate and read. Additionally, if there are changes to the User model, the developer can easily modify or add new tests for each method by editing the corresponding test class.
Conclusion
When it comes to organizing tests in Python, using one test class per method offers several advantages over testing multiples methods in one test class. This approach makes it easier for developers to keep tests organized and maintain them effectively. With each method tested in its own class, developers can quickly determine what each test is testing. Plus, it ensures that each test focuses on a specific functionality of the code being tested.