Web UI testing is a tricky thing in many aspects. They requires many dependencies, they are slow and they are often flaky. In this post I would like to share my web UI testing experience on an example of one python library.


I maintain a library that Red Hat uses for web UI testing. It’s called widgetastic.patternfly4 and you can find it on Github. It abstracts some components from Patternfly design system.


Every good library should have tests otherwise we cannot even know if something work there. Therefore PR testing is a “must have” nowadays. There are many cloud base CI systems and I chose Travis CI. I’ve already had some experience with it and that was a main reason of my choice. All of these CI services give you a virtual machine where you can run your code. In the most of cases preinstalled software is enough. The real fun begins when you need to install and setup something that is missing.

This is a case of widgetastic.patternfly4. It has selenium in the dependencies. It means that before a test session starts we need to install a browser (Chrome or Firefox) and an appropriate webdriver (geckodriver or chromedriver). The very straightforward solution would be installing all that stuff during the PR testing. My Travis CI config looked something like this at that time:

# Travis CI provides an easy way to install a browser
  firefox: latest
language: python
stage: test
env: BROWSER=firefox
  - pip install -U setuptools pip
  - pip install flake8 pytest pytest-cov coveralls
  # I found an utility that downloads a right version of the webdriver
  - pip install webdriverdownloader
  - webdriverdownloader firefox
install: python setup.py install
  - flake8 src testing --max-line-length=100
  - py.test -v --cov widgetastic_patternfly4 --cov-report term-missing


The aforementioned approach didn’t work well. First it takes time to install all dependencies. Second webdriverdownloader was buggy and often tests couldn’t even start. Another issue was in the way how browsers run. Obviously CI machine doesn’t need any GUI therefore we have to run both Chrome and Firefox in headless mode. In this mode the browser renders the pages in the memory. It turned out that test results may differ significantly.

The solution was on the surface. Containers was already the industry standard and they perfectly fit to my problem. Now I’m thinking why I didn’t come up to it from the beginning 😅. So instead of installing dependencies during PR testing it’s better to pack everything what we need in a one container image. I created such image and pushed it to quay.io image registry. After that CI config became into:

language: python
stage: test
# here we start docker service
  - docker
env: BROWSER=firefox
  # The container has Selenium standalone running. In the tests we can use Remote webdriver to
  # connect to it. Moreover we can even have X server inside and run browsers in regular way.
  - docker run -d -p 4444:4444 -v /dev/shm:/dev/shm quay.io/redhatqe/selenium-standalone
  - pip install -U setuptools pip wheel
  - pip install pytest pytest-cov codecov
  - python setup.py install
script: pytest -v --no-cov-on-fail --cov=widgetastic_patternfly4

Can I do better?

I was satisfied for while and switched to other projects. In the meantime the library was growing. More components were added and tests as well. A session of 116 tests took more than 20 minutes. I wanted to do something about it. It needs to know that tests are run sequentially, one by one. This is what I tried to experiment with. pytest has a very nice plugin called pytest-xdist. It can distribute tests among some number of workers. Ok, let’s try it:

$ pip install pytest-xdist
$ pytest -v -n 8

It just started to execute the tests in parallel! But in my setup all tests were executed against one container. That caused undesirable side effects and may tests failed. So we need to give a dedicated container to each worker. For that I added a fixture that spawns a container on some local host address and then python selenium uses it to interact with a browser. Here is the fixture:

def selenium_host(worker_id):
    oktet = 1 if worker_id == "master" else int(worker_id.lstrip("gw")) + 1
    host = f"127.0.0.{oktet}"
    ps = subprocess.run(
    yield host
    container_id = ps.stdout.decode("utf-8").strip()
    subprocess.run(["sudo", "podman", "kill", container_id])

As you can see I just get a worker id and make a host address from it. I use podman to manage containers. Unfortunately, version 2.0.2 has a nasty bug that breaks networking in rootless containers. So I had to run containers with sudo but I’ll remove it when the bug will be fixed. Local experiments went fine. The next task is to prepare the CI system. As I told earlier I was using Travis CI and it has one drawback. It doesn’t have podman preinstalled. Strictly speaking nothing could prevent me to use docker but I prefer podman due to its daemonless approach and rootless containers.

In the end I switched to Github Actions. Their instances have podman preinstalled and Actions have a nice feature of shared steps. But in other aspects I wouldn’t highlight that CI system among others. Here is an excerpt of workflow.yaml:

  runs-on: ubuntu-latest
  needs: lint
      browser: ["firefox", "chrome"]
    - uses: actions/checkout@v2
    - uses: actions/setup-python@v2
    - name: Install dependencies
      run: |
        pip install -U setuptools pip
        pip install pytest pytest-cov codecov pytest-xdist
        python setup.py install
    - name: Test with pytest
        BROWSER: $
      # Containers are spawn from the tests now.
      # "-n 5" means that 5 workers are used with 5 dedicated containers.
      # "--dist=loadscope" means tests spread among workers by modules. Each worker executes tests
      # from the same module.
      run: |
        pytest -v -n 5 --dist=loadscope --no-cov-on-fail --cov=widgetastic_patternfly4 --cov-report=xml:/tmp/coverage.xml
    - name: Publish coverage
      uses: codecov/codecov-action@v1
        file: /tmp/coverage.xml

So what about time execution? It’s reduced to 11 minutes and it’s two times faster! Not bad. Can I do better?