2: Pytest testing framework#

This lesson explains how to use the pytest testing framework to make testing easier.

Other lessons in this tutorial:#

This lesson covers:#


The example plugin and all the tests discussed in this lesson are available in this GitHub repository.


We are using pytest as a testing framework. It provides convenience tools to assist with testing. For example, it can discover tests for you if you point it to a directory or a file. It can be installed using pip install pytest.

Testing framework features#

Testing frameworks provide a whole host of useful features, including:

  • Test discovery - directories can be crawled (searched) to find things that look like tests and run them

  • Housekeeping and ease of use - convenient methods for writing tests and cleaning up after running the tests

Pytest goes through the target destination, such as a file or directory, finding any method or function prefaced with the word test. It runs all the methods and functions prefaced with the word test but not the code under the main block. When pytest runs against example_test.py (refer to the Python’s assert keyword lesson), it finds several tests that all pass.

If the tests fail, pytest is very good at tracing back the reason they failed and showing their values throughout test execution. In more complicated examples, this traceback mechanism can be very helpful. In this example, the message is that we got a Pass but were expecting a Fail. See the lines below that show the assert keyword and the errors.

(napari-env) user@directory % pytest   

========================== test session starts ==========================  
Platform darwin -- 3.9.7, pytest-6.2.5, py-1.11.0, pluggy-1.0.0  
PyQt5 5.15.6 -- Qt runtime 5.15.2 -- Qt compiled 5.15.2  
rootdir: /Users/user/directory  
Plugins: qt-4.0.2, napari-plugin-engine-0.2.0, napari-0.4.13  
Collected 2 items  

example_func.py .F  
================================= FAILURES ===============================  
—-------------------------- test_get_grade_fail --------------------------  
def test_get_grade_fail():  
    grade = get_grade_from_mark(65)  
>   assert grade == “Fail”, f”Expected 65 to fail but result was {grade}”  
E   AssertionError: Expected 65 to fail but result was Pass  
E   assert ‘Pass’ == ‘Fail’  
E     - Fail  
E     + Pass  
example_func_py: 16: AssertionError  
======================== short test summary info =========================  
FAILED example_func_py::test_get_grade_fail - AssertionError: Expected 65 to fail, but result was Pass  
====================== 1 failed in 0.56s =======================  
(napari-env) user@directory %   


Another very useful tool that pytest provides is parametrization.

We’ve tested these functions with a single value. We need to be more thorough. Pytest allows us to parametrize tests. We decorate our function with @pytest.mark.parametrize and pass the decorator a parameter name, mark, as a string, and a list of values for which we’d like to run the test function. Note that we pass in 50 as an edge case; it’s the lowest mark that will pass.

import pytest

def get_grade_from_mark(mark):
    if mark > 50
        return "Pass"
        return "Fail"

@pytest.mark.parametrize("mark", [65, 80, 50])
def test_get_grade_pass(mark):
    grade = get_grade_from_mark(mark):
    assert grade == "Pass", f"Expected {mark} to pass, but result was{grade}"

@pytest.mark.parametrize("mark", [40, 25])
def test_get_grade_fail(mark):
    grade = get_grade_from_mark(mark):
    assert grade == "Fail", f"Expected {mark} to fail, but result was{grade}"

if _name_ == "_main_";
    print("All passing.")

We run pytest which finds and runs test_get_grade_pass(mark). test_get_grade_pass(mark) fails because the code says mark > 50, so a mark of 50 still fails. The code should say >=50 for a mark to pass.

example_func.py ..F..
================================= FAILURES ===============================  
—------------------------ test_get_grade_pass[50] ------------------------  
mark = 50  
@pytest parametrize(‘mark’, [65, 80, 50])  
def test_get_grade_pass(mark):  
    grade = get_grade_from_mark(mark)  
>   assert grade == “Pass”, f”Expected {mark} to pass, but result was {grade}”  
E   AssertionError: Expected 50 to pass but result was fail  
E   assert ‘Fail’ == ‘Pass’  
E     - Pass  
E     + Fail  

example_func_py:12: AssertionError  
======================== short test summary info =========================  
FAILED example_func_py::test_get_grade_pass[50] - AssertionError: Expected 50 to pass, but result was Fail   

Another valuable feature of pytest is the pytest-cov option discussed in the Test coverage lesson

The next lesson in this tutorial on testing is the Readers and fixtures lesson.