Grouping tests in pytest: Classes vs plain functions

Question

I'm using pytest to test my app. pytest supports 2 approaches (that I'm aware of) of how to write tests:

1. In classes:

test_feature.py -> class TestFeature -> def test_feature_sanity

2. In functions:

test_feature.py -> def test_feature_sanity

Is the approach of grouping tests in a class needed? Is it allowed to backport unittest builtin module? Which approach would you say is better and why?

Answer 1

There are no strict rules regarding organizing tests into modules vs classes. It is a matter of personal preference. Initially I tried organizing tests into classes, after some time I realized I had no use for another level of organization. Nowadays I just collect test functions into modules (files).

I could see a valid use case when some tests could be logically organized into same file, but still have additional level of organization into classes (for instance to make use of class scoped fixture). But this can also be done just splitting into multiple modules.

Answer 2

This answer presents two compelling use-cases for a TestClass in pytest:

• Joint parametrization of multiple test methods belonging to a given class.
• Reuse of test data and test logic via subclass inheritance

Joint parametrization of multiple test methods belonging to a given class.

The pytest parametrization decorator, @pytest.mark.parametrize, can be used to make inputs available to multiple methods within a class. In the code below, the inputs param1 and param2 are available to each of the methods TestGroup.test_one and TestGroup.test_two.

"""test_class_parametrization.py""" import pytest  @pytest.mark.parametrize(     "param1,param2",     [         ("a", "b"),         ("c", "d"),     ], ) class TestGroup:     """A class with common parameters, `param1` and `param2`."""      @pytest.fixture     def fixt(self):         """This fixture will only be available within the scope of TestGroup"""         return 123      def test_one(self, param1, param2, fixt):         print("\ntest_one", param1, param2, fixt)      def test_two(self, param1, param2):         print("\ntest_two", param1, param2) 

$ pytest -s test_class_parametrization.py ================================================================== test session starts ================================================================== platform linux -- Python 3.8.6, pytest-6.2.1, py-1.10.0, pluggy-0.13.1 rootdir: /home/jbss plugins: pylint-0.18.0 collected 4 items  test_class_parametrization.py test_one a b 123 . test_one c d 123 . test_two a b . test_two c d .  =================================================================== 4 passed in 0.01s =================================================================== 

Reuse of test data and test logic via subclass inheritance

I'll use a modified version of code taken from another answer to demonstrate the usefulness of inheriting class attributes/methods from TestClass to TestSubclass:

# in file `test_example.py` class TestClass:     VAR = 3     DATA = 4      def test_var_positive(self):         assert self.VAR >= 0   class TestSubclass(TestClass):     VAR = 8      def test_var_even(self):         assert self.VAR % 2 == 0      def test_data(self):         assert self.DATA == 4 

Running pytest on this file causes four tests to be run:

$ pytest -v test_example.py =========== test session starts =========== platform linux -- Python 3.8.2, pytest-5.4.2, py-1.8.1 collected 4 items  test_example.py::TestClass::test_var_positive PASSED test_example.py::TestSubclass::test_var_positive PASSED test_example.py::TestSubclass::test_var_even PASSED test_example.py::TestSubclass::test_data PASSED 

In the subclass, the inherited test_var_positive method is run using the updated value self.VAR == 8, and the newly defined test_data method is run against the inherited attribute self.DATA == 4. Such method and attribute inheritance gives a flexible way to re-use or modify shared functionality between different groups of test-cases.