Running tests in containers in Docker - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

Output of a Docker test run command

What is the output of running this Docker command to execute tests inside a container?

Docker

docker run --rm -v $(pwd):/app -w /app python:3.12 python -m unittest discover tests

A Error: No such file or directory: tests

B Ran 5 tests in 0.002s\n\nOK

C SyntaxError: invalid syntax in test file

D Permission denied while accessing /app

💡 Hint

The command mounts the current directory and runs Python tests inside the container.

The command mounts the current directory into /app in the container, sets the working directory to /app, and runs Python unittest discovery on the 'tests' folder. If tests exist and run successfully, it outputs the number of tests run and 'OK'.

🧠 Conceptual

intermediate

Understanding Docker test container isolation

Why is it beneficial to run tests inside Docker containers instead of directly on the host machine?

A It ensures tests run in a consistent environment regardless of the host setup.

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

C It automatically fixes bugs in the test code.

D It disables network access for all tests by default.

💡 Hint

Think about environment differences between machines.

Running tests in containers isolates them from the host environment, ensuring consistent dependencies and configurations. This avoids issues caused by differences in host setups.

❓ Configuration

advanced

Dockerfile for running tests with environment variables

Which Dockerfile snippet correctly sets an environment variable and runs tests using pytest inside the container?

A

FROM python:3.12
ENV TEST_ENV=ci
WORKDIR /app
COPY . .
RUN pip install pytest
CMD ["pytest", "tests"]

B

FROM python:3.12
WORKDIR /app
COPY . .
RUN pip install pytest
CMD ["TEST_ENV=ci pytest tests"]

C

FROM python:3.12
WORKDIR /app
COPY . .
RUN pip install pytest
ENV TEST_ENV=ci
CMD ["pytest", "tests"]

D

FROM python:3.12
WORKDIR /app
COPY . .
RUN pip install pytest
CMD pytest tests
ENV TEST_ENV=ci

💡 Hint

Environment variables should be set before the command runs.

Option A correctly sets the environment variable TEST_ENV before running the tests. ENV must come before CMD to be effective. CMD uses exec form to run pytest.

❓ Troubleshoot

advanced

Diagnosing test failures in Docker container

You run tests inside a Docker container but get this error: "ModuleNotFoundError: No module named 'requests'". What is the most likely cause?

A The container does not have network access to download tests.

B The tests are running on the host, not inside the container.

C The Dockerfile is missing the WORKDIR instruction.

D The 'requests' package is not installed inside the container.

💡 Hint

Think about Python dependencies inside containers.

The error means the Python package 'requests' is missing inside the container environment. It must be installed via pip in the Dockerfile or before running tests.

🔀 Workflow

expert

Best workflow to run tests in CI with Docker

Which sequence of steps is the best workflow to run automated tests in a CI pipeline using Docker containers?

A 4,2,1,3

B 1,4,2,3

C 4,1,2,3

D 1,2,4,3

💡 Hint

Consider the logical order of preparing images, running tests, and pushing images.

First pull the latest base image (4) to get updates, then build the image (1), run tests inside the container (2), and if tests pass, push the image to the registry (3).

Practice

1. What is the main benefit of running tests inside Docker containers?

easy

A. It creates a clean and consistent test environment isolated from your local machine.

B. It makes tests run faster than on any other system.

C. It automatically fixes bugs in your code during testing.

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

Solution

1. Step 1: Understand container isolation

   Docker containers run applications in isolated environments separate from the host system.

2. Step 2: Benefits for testing

   This isolation ensures tests run in a clean, consistent environment every time, avoiding conflicts with local setup.

3. Final Answer:

   It creates a clean and consistent test environment isolated from your local machine. -> Option A

4. Quick Check:

   Container isolation = clean test environment [OK]

Hint: Think: containers keep tests separate from your computer [OK]

Common Mistakes:

• Assuming containers speed up tests automatically
• Believing Docker fixes code bugs
• Thinking tests run without commands

2. Which of the following docker run commands correctly runs tests inside a container and removes the container after completion?

easy

A. docker run -it myapp pytest

B. docker run --name testcontainer myapp pytest

C. docker run -d myapp pytest

D. docker run --rm myapp pytest

Solution

1. Step 1: Understand the --rm flag

   The --rm option tells Docker to remove the container after it finishes running.

2. Step 2: Match command to requirement

   docker run --rm myapp pytest uses --rm and runs pytest inside the container, which fits the question.

3. Final Answer:

   docker run --rm myapp pytest -> Option D

4. Quick Check:

   --rm removes container after run [OK]

Hint: Use --rm to auto-remove containers after tests [OK]

Common Mistakes:

• Using -d runs container detached, not suitable for tests
• Forgetting --rm leaves stopped containers
• Using --name does not remove container automatically

3. Given this command:
docker run --rm -v $(pwd):/app -w /app python:3.12 pytest test_sample.py
What will happen when you run it?

medium

A. Mounts the current directory but runs tests on the host machine, not inside the container.

B. Runs pytest on test_sample.py inside the container using the current directory code, then removes the container.

C. Starts a container but does not run any tests because pytest is missing.

D. Creates a container but keeps it running after tests finish.

Solution

1. Step 1: Analyze volume mount and working directory

   The -v $(pwd):/app mounts current folder to /app in container; -w /app sets working directory inside container.

2. Step 2: Understand command execution

   The container runs pytest test_sample.py inside /app, using the mounted code, then --rm removes container after.

3. Final Answer:

   Runs pytest on test_sample.py inside the container using the current directory code, then removes the container. -> Option B

4. Quick Check:

   Volume mount + pytest run + --rm = Runs pytest on test_sample.py inside the container using the current directory code, then removes the container. [OK]

Hint: Mount code and run tests inside container with --rm [OK]

Common Mistakes:

• Thinking tests run on host, not inside container
• Assuming container stays after tests
• Ignoring volume mount effect

4. You run this command to test your app:
docker run -v $(pwd):/app -w /app python:3.12 pytest
But the container stays running after tests finish. What is the likely cause?

medium

A. The -w flag causes the container to hang.

B. The -v flag is incorrect and prevents container exit.

C. You forgot to add the --rm flag to remove the container after running.

D. pytest command is missing, so container never runs tests.

Solution

1. Step 1: Check container removal flags

   Without --rm, Docker keeps the container stopped but present after run.

2. Step 2: Verify other flags

   Volume mount and working directory flags do not affect container removal behavior.

3. Final Answer:

   You forgot to add the --rm flag to remove the container after running. -> Option C

4. Quick Check:

   Missing --rm = container stays [OK]

Hint: Add --rm to auto-remove containers after tests [OK]

Common Mistakes:

• Blaming volume mount for container state
• Thinking working directory affects container lifecycle
• Assuming pytest missing causes container to stay

5. You want to run tests inside a container using your local code, but your tests require environment variables API_KEY and DEBUG=true. Which command correctly runs tests with these variables and cleans up after?

hard

A. docker run --rm -v $(pwd):/app -w /app -e API_KEY=12345 -e DEBUG=true python:3.12 pytest

B. docker run --rm -v $(pwd):/app -w /app -e API_KEY -e DEBUG=true python:3.12 pytest

C. docker run -v $(pwd):/app -w /app -e API_KEY=12345 -e DEBUG=true python:3.12 pytest

D. docker run --rm -v $(pwd):/app -w /app python:3.12 pytest API_KEY=12345 DEBUG=true

Solution

1. Step 1: Set environment variables correctly

   Use -e VAR=value to pass environment variables with values into the container.

2. Step 2: Ensure container cleanup

   The --rm flag removes the container after tests finish, keeping environment clean.

3. Final Answer:

   docker run --rm -v $(pwd):/app -w /app -e API_KEY=12345 -e DEBUG=true python:3.12 pytest -> Option A

4. Quick Check:

   Use -e VAR=value and --rm for env vars and cleanup [OK]

Hint: Pass env vars with -e VAR=value and add --rm [OK]

Common Mistakes:

• Passing -e without values
• Omitting --rm causing leftover containers
• Appending env vars as command arguments