Optimizing Test Execution: Running live_server Tests Last with Pytest

When working with Django applications, it's common to have a mix of fast unit tests and slower end-to-end (E2E) tests that use Pytest's live_server fixture and browser automation tools like Playwright or Selenium. To ensure my test suite runs efficiently, I want to execute the slower tests at the end. In this blog post, I will walk you through how to configure pytest to run these slow tests last.

Why Run Slow Tests Last?

Running slow tests last has several advantages:

• Efficiency: Faster feedback from unit tests allows you to catch and fix issues early.
• Resource Management: Keeps the resources consumed by slow tests (like database connections or external service calls) to a minimum until necessary.

1. Setting Up Pytest with Django and Playwright

First, let's ensure you have pytest, pytest-django, and Playwright installed:

pip install pytest pytest-django pytest-playwright
playwright install

2. Defining Custom Markers

Markers in pytest allow you to tag your tests, making it easier to manage and run specific subsets. We'll define a custom marker e2e to mark all tests using live_server automatically.

Create or update your pytest.ini file:

[pytest]
DJANGO_SETTINGS_MODULE = myproject.settings
markers =
    e2e: marks tests that use Django's live server (deselect with '-m "not e2e"')

3. Writing a Custom Pytest Plugin

To control the order in which tests are run, we will write a custom pytest plugin. This plugin will automatically mark tests that use live_server fixtures with the e2e marker and ensure that marked tests run after all other tests.

Create or update conftest.py in your project root:

def pytest_collection_modifyitems(items):
    live_server_tests = []
    other_tests = []

    for item in items:
        if "live_server" in getattr(item, "fixturenames", ()):
            item.add_marker("e2e")
            live_server_tests.append(item)
        else:
            other_tests.append(item)

    # Modify the items list to run live_server tests last
    items[:] = other_tests + live_server_tests

4. Adding Example Tests

Create a test.py file in your project and add the following:

import pytest


def test_e2e_test(live_server, page):
    page.goto(live_server.url)
    assert page.evaluate("document.title") == "Homepage | Sitename"


def test_simple_unit_test():
    assert 1 + 1 == 2

5. Running Your Tests

Now, when you run pytest, the tests will be executed in the desired order: the unit test first and then the end-to-end test.

pytest

You should see your fast unit tests run first, followed by the slower tests that use live_server and Playwright/Selenium.

If you want to run only unit tests, excluding slow tests marked with the e2e tag, run pytest like this:

pytest -m "not e2e"

That’s it! I wish you happy and thorough testing!