Selenium + Pytest tests run in a Docker container
Testing with Selenium can be tricky some times. Interacting with the DOM might be hard and it’s full of undesired
behaviours.
One big problem when it comes to testing with Selenium is versioning and environment. Running tests with a specific
version of Firefox could work but it won’t for another. These tests work on my Mac but they don’t on this other Debian. The Python version I’m using on this host is different to yours.
Docker is the king of fixing this kind of problems. I though it could be a good idea to create a scenario where
tests were running faithfully and reliably.
Everything here has be tested on Elementary OS 5.1.7 Hera (based on Ubuntu 18.04). I’ve used
Docker 19.03.12-19.03.13, Docker Compose 1.26.1 and Python 3.8.3-3.9.0.
I’ve developed a small testing application under selenium_tests/. It uses Pytest as testing framework.
The tests are developed implementing a page object pattern.
They are run against the web http://automationpractice.com, which seems to be a webpage to use in cases like this.
We are using Docker, so there’s no need of running the tests locally. But if you really want it, here it is a how to:
1) Make sure you are on the selenium_tests/ root.
2) Set a Python virtual environment. Activate it.
3) Install all the dependencies defined on selenium_tests/requirements.txt (pip install -r requirements.txt).
4) Download the last version of the Geckodriver (we will use Firefox as browser, so we assume you got it install on your system).
Running the following command will work:wget -q "https://github.com/mozilla/geckodriver/releases/download/v0.26.0/geckodriver-v0.26.0-linux64.tar.gz" -O /tmp/geckodriver.tgz && tar zxf /tmp/geckodriver.tgz -C ./selenium_tests/ && rm /tmp/geckodriver.tgz
5) Finally, simply run: pytest
They complement each other. While the selenium/hub deploys a container enabling a Selenium Grid, the
selenium/node-firefox uses that grid and deploys a node with Firefox so tests can work using that browser.
A VNC server is also installed. To sum up, they will create the infrastructure where the tests will be run against.
You can read all the documentation about these images here.
This image is defined on this repo Dockerfile. It consists on a python:3.9.0-alpine Docker image meaning that
the deployed container will be a very lightweight system (barely 100mB) with that Python version installed.
The image copies the contents of the selenium_tests/ directory and install the Python packages defined onselenium_tests/requirements.txt, so we set a perfect environment where to run the tests. It also downloads a script
named wait-for-it.sh and save it on the working directory. We’ll talk about this on the following section.
Although as we’ll see later there’s no need, you could manually build an image by running the following on the repo root.
docker build -t "pytest-host:latest" .
A docker-compose.yml is been also developed for making all the deployment process much easier. The three images
discussed above are defined as services here.
First of all the pytest-host image is built. The selenium/hub and selenium/node-firefox containers are deployed
together. The first one exposes the port TCP/4444, where the grid is running. The pytest-host container is waiting for the
selenium/node-firefox port 5555 to be ready to run the tests. This wait isn’t natively implemented on Docker Compose.
That’s why it’s necessary to use third party scripts as the already mentioned
wait-for-it.sh.
A bridge type network is also created and linked to all services so we make sure they all share the same network.
This is not strictly necessary since by default all services deployed using the same docker-compose project belongs to
the same network.
There are several ways of running the tests using the developed Docker infrastructure.
This is easily the simplest way of running your tests. Once your test files are up to date, you can run
docker-compose up --abort-on-container-exit --remove-orphans
The pytest-host image will always be built so new changes will be implemented. Then all the containers will be
deployed and the tests run.
Once you’ve finished working on this, run:
docker-compose down && docker system prune -f --volumes
Your system status will be the same as before starting the process.
To make things even more easier, I’ve created a Makefile with some helpful rules defined. Run make run-tests-and-clean
and all tests will run and then the environment will be cleaned up. This is the option I highly encourage to use.
If you would like to see what it’s happening on the Docker, you can use the “debug” mode. Just edit
file .env, set DEBUG to -debug and then follow this instructions here.
Let’s suppose you want to take advantage of this process but you don’t need all the tests to be run.
Using our example, we will execute only the tests contained under the module selenium_tests/tests/tests_searches.py.
Run the following:
docker-compose run --rm pytest-host tests/test_searches.py
If you ran the docker-compose up command before and receive an error message similar toERROR: for selenium-hub Cannot create container for service selenium-hub: Conflict, stop and remove all the
containers involved. If you rather want to remove all the active containers, run:
docker stop $(docker ps -aq) && docker rm $(docker ps -aq)
By default, when running the tests with docker-compose up, they will be executed in parallel two at the time.
You can change this by modifying the -n flag of the pytest command.