Docker-based test execution in PyTest - Build an Automation Script

Run pytest tests inside a Docker container

Preconditions (3)

Step 1: Create a Dockerfile that installs Python and pytest

Step 2: Copy the project files into the Docker image

Step 3: Build the Docker image with a tag 'pytest-test-image'

Step 4: Run a container from the image executing 'pytest' command

Step 5: Observe the test output in the console

✅ Expected Result: Tests run inside the Docker container and pytest output shows all tests passing or failing as expected

Automation Requirements - pytest

Assertions Needed:

Verify that pytest command runs successfully inside the container

Verify that test results are output to the console

Best Practices:

Use a Dockerfile to define the test environment

Use explicit commands to build and run the Docker container

Keep the Docker image lightweight by using official Python base images

Mount the project directory as a volume for easy test code updates

Automated Solution

PyTest

import subprocess
import sys


def test_run_pytest_in_docker():
    # Build the Docker image
    build_cmd = [
        "docker", "build", "-t", "pytest-test-image", "."
    ]
    build_process = subprocess.run(build_cmd, capture_output=True, text=True)
    assert build_process.returncode == 0, f"Docker build failed: {build_process.stderr}"

    # Run the Docker container to execute pytest
    run_cmd = [
        "docker", "run", "--rm", "pytest-test-image"
    ]
    run_process = subprocess.run(run_cmd, capture_output=True, text=True)
    print(run_process.stdout)
    assert run_process.returncode == 0, f"Pytest failed inside Docker: {run_process.stdout}\n{run_process.stderr}"


if __name__ == "__main__":
    test_run_pytest_in_docker()

This script automates running pytest tests inside a Docker container.

First, it builds a Docker image named pytest-test-image using the Dockerfile in the current directory. It checks if the build succeeded by verifying the return code.

Next, it runs a container from this image, which executes pytest tests. The output is captured and printed to the console.

Assertions check that both the Docker build and the pytest run complete successfully. If any step fails, the test fails with an error message showing the problem.

This approach ensures tests run in a clean, isolated environment defined by the Dockerfile.

Common Mistakes - 4 Pitfalls

Not checking the return code of the Docker build or run commands

Hardcoding absolute paths inside the Dockerfile or run command

Not printing or capturing the pytest output

Using a large base image without reason

Bonus Challenge

Now add data-driven testing by running pytest inside Docker with three different Python versions: 3.10, 3.11, and 3.12

Show Hint

Practice

1. What is the main benefit of running pytest tests inside a Docker container?

easy

A. It ensures tests run in a clean, consistent environment every time.

B. It makes tests run faster than on the host machine.

C. It automatically fixes failing tests.

D. It allows tests to run without writing any test code.

Solution

1. Step 1: Understand Docker container purpose

   Docker containers provide isolated environments that are the same every time they run.

2. Step 2: Connect to pytest benefits

   Running pytest inside Docker avoids environment differences causing test failures.

3. Final Answer:

   It ensures tests run in a clean, consistent environment every time. -> Option A

4. Quick Check:

   Docker isolation = consistent test environment [OK]

Hint: Docker isolates environment to avoid setup issues [OK]

Common Mistakes:

• Thinking Docker speeds up tests automatically
• Believing Docker fixes test code errors
• Assuming tests run without writing code

2. Which of the following is the correct way to run pytest inside a Docker container with your current directory shared?

easy

A. docker run -p 8080:80 python:3.12 pytest

B. docker run -v $(pwd):/app -w /app python:3.12 pytest

C. docker build -t pytest-image . && docker run pytest-image

D. docker run --rm python:3.12 pytest /app

Solution

1. Step 1: Identify volume sharing syntax

   The -v $(pwd):/app option shares the current directory to /app inside the container.

2. Step 2: Check working directory and command

   -w /app sets the working directory, and pytest runs tests there.

3. Final Answer:

   docker run -v $(pwd):/app -w /app python:3.12 pytest -> Option B

4. Quick Check:

   Volume + workdir + pytest = correct run command [OK]

Hint: Use -v for volume and -w for working directory [OK]

Common Mistakes:

• Using -p instead of -v for volume
• Not setting working directory with -w
• Running pytest without mounting code

3. Given this Docker command:

docker run --rm -v $(pwd):/tests -w /tests python:3.12 pytest test_sample.py

What will happen if test_sample.py contains one passing and one failing test?

medium

A. Pytest will run only the passing test and ignore failures.

B. Pytest will skip the failing test and exit with zero code.

C. Docker will fail to start because of volume issues.

D. Pytest will run both tests and exit with a non-zero code indicating failure.

Solution

1. Step 1: Understand pytest behavior on failures

   Pytest runs all tests and returns a non-zero exit code if any test fails.

2. Step 2: Confirm Docker command mounts tests correctly

   The volume mounts current directory to /tests, so test_sample.py is accessible and runs.

3. Final Answer:

   Pytest will run both tests and exit with a non-zero code indicating failure. -> Option D

4. Quick Check:

   Failing test causes pytest exit code != 0 [OK]

Hint: Pytest exit code shows if tests passed or failed [OK]

Common Mistakes:

• Assuming pytest skips failing tests
• Thinking Docker volume mount fails here
• Believing pytest ignores failures

4. You run this command:

docker run -v $(pwd):/app python:3.12 pytest

But pytest does not find any tests. What is the most likely cause?

medium

A. You did not set the working directory inside the container.

B. The Python image does not have pytest installed.

C. Docker volume syntax is incorrect.

D. Tests must be named test_docker.py to run.

Solution

1. Step 1: Check working directory usage

   Without -w /app, pytest runs in root, not where tests are mounted.

2. Step 2: Understand pytest test discovery

   Pytest looks for tests in current directory; wrong directory means no tests found.

3. Final Answer:

   You did not set the working directory inside the container. -> Option A

4. Quick Check:

   Working directory missing = no tests found [OK]

Hint: Always set -w to your mounted test folder [OK]

Common Mistakes:

• Assuming pytest is missing in python image by default
• Blaming volume syntax without error
• Thinking test file names must be exact

5. You want to run pytest inside Docker and save the test report to your host machine. Which command correctly achieves this?

hard

A. docker run -v $(pwd):/app -w /app python:3.12 pytest --junitxml=/tmp/report.xml

B. docker run python:3.12 pytest --junitxml=/app/report.xml

C. docker run -v $(pwd):/app -w /app python:3.12 pytest --junitxml=/app/report.xml

D. docker run -v $(pwd):/app python:3.12 pytest --junitxml=report.xml

Solution

1. Step 1: Ensure volume mount and working directory

   Mount current directory to /app and set working directory to /app so files are accessible.

2. Step 2: Specify report path inside container

   Use --junitxml=/app/report.xml to save report inside mounted folder, making it visible on host.

3. Final Answer:

   docker run -v $(pwd):/app -w /app python:3.12 pytest --junitxml=/app/report.xml -> Option C

4. Quick Check:

   Volume + workdir + report path inside mount = correct [OK]

Hint: Set -w and use full path for report inside mounted folder [OK]

Common Mistakes:

• Not setting working directory with -w
• Using relative path without mount inside container
• Omitting volume mount so report is lost