Docker-based test execution in PyTest - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of pytest run inside Docker container

Consider a Docker container running a pytest suite with the following test file test_sample.py:

def test_addition():
    assert 2 + 3 == 5

def test_subtraction():
    assert 5 - 3 == 1

What will be the result of running pytest inside the container?

PyTest

def test_addition():
    assert 2 + 3 == 5

def test_subtraction():
    assert 5 - 3 == 1

A 2 passed

B 0 tests collected

C 2 failed

D 1 passed, 1 failed

💡 Hint

Check the assertions carefully for correctness.

The first test passes because 2 + 3 equals 5. The second test fails because 5 - 3 equals 2, not 1.

❓ locator

intermediate

Best locator for pytest test file in Docker context

You want to run pytest inside a Docker container. Which file path locator is best to specify the test file test_api.py located in the /app/tests/ directory inside the container?

A ./tests/test_api.py

B /app/tests/test_api.py

C /tests/test_api.py

D tests/test_api.py

💡 Hint

Inside Docker, absolute paths are more reliable if you know the working directory.

Using the absolute path /app/tests/test_api.py ensures pytest finds the test file regardless of the current working directory inside the container.

❓ assertion

advanced

Assertion outcome in Docker pytest log

Inside a Docker container, a pytest test contains:

def test_string():
    result = 'hello'.upper()
    assert result == 'HELLO'

What will be the assertion outcome when running pytest inside Docker?

PyTest

def test_string():
    result = 'hello'.upper()
    assert result == 'HELLO'

A Test raises TypeError

B Test fails with AssertionError

C Test passes

D Test is skipped

💡 Hint

Check the string method and expected value carefully.

The upper() method converts 'hello' to 'HELLO', so the assertion is true and test passes.

🔧 Debug

advanced

Debugging pytest failure inside Docker

A pytest test inside Docker fails with:

FileNotFoundError: [Errno 2] No such file or directory: '/data/input.txt'

What is the most likely cause?

A The file /data/input.txt is missing inside the Docker container

B Docker container has no network access

C The test code has a syntax error

D pytest is not installed in the container

💡 Hint

FileNotFoundError means the file path is invalid or file is missing.

The error indicates the test tries to open a file that does not exist inside the container's filesystem.

❓ framework

expert

Choosing Docker test execution strategy for pytest

You want to run pytest tests inside Docker with these requirements:
- Tests must run in isolated containers
- Test results should be collected on host machine
- Tests should run in parallel

Which Docker strategy best fits these requirements?

A Use Docker Compose to spin up multiple containers running pytest in parallel, mounting a shared volume for test reports

B Run pytest directly on host without Docker for faster execution

C Use a single Docker container running pytest sequentially with no volume mounts

D Run pytest inside Docker but do not collect test reports

💡 Hint

Think about isolation, parallelism, and sharing results between container and host.

Docker Compose can orchestrate multiple containers for parallel test runs. Mounting a shared volume allows test reports to be accessible on the host.

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