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).