Running tests in containers in Docker - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the command to run tests inside a Docker container.

Docker

docker run --rm [1] myapp:test

Drag options to blanks, or click blank then click option'

A--name

B-d

C--rm

D-it

Attempts:

3 left

💡 Hint

Common Mistakes

Using --rm twice causes an error.

Using -d runs container in background, hiding test output.

The -it option allows interactive terminal access, which is useful for running tests inside the container.

2fill in blank

medium

Complete the command to mount the current directory inside the container for testing.

Docker

docker run --rm -v [1]:/app myapp:test

Drag options to blanks, or click blank then click option'

A$(pwd)

B/home/user/project

C/app

D/var/lib/docker

Attempts:

3 left

💡 Hint

Common Mistakes

Hardcoding paths can cause errors on different machines.

Using container paths instead of host paths for volume mount.

$(pwd) dynamically gets the current directory path to mount inside the container.

3fill in blank

hard

Fix the error in the command to run tests with environment variables.

Docker

docker run --rm -e [1] myapp:test

Drag options to blanks, or click blank then click option'

AENV

BENV=production

CENV=production debug=true

DENV production

Attempts:

3 left

💡 Hint

Common Mistakes

Using space instead of equals sign causes syntax error.

Passing multiple variables in one -e option is invalid.

The -e option requires KEY=VALUE format for environment variables.

4fill in blank

hard

Fill both blanks to run tests in detached mode and name the container.

Docker

docker run [1] [2] myapp:test

Drag options to blanks, or click blank then click option'

A-d

B--rm

C--name test_container

D-it

Attempts:

3 left

💡 Hint

Common Mistakes

Using -it with detached mode causes conflicts.

Forgetting to name the container makes management harder.

-d runs the container in detached mode, and --name assigns a custom container name.

5fill in blank

hard

Fill all three blanks to build an image, tag it, and run tests inside the container.

Docker

docker build -t [1] . && docker run --rm -v [2]:/app [3] myapp:test

Drag options to blanks, or click blank then click option'

Amyapp:test

B$(pwd)

D-it

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing image tag and container name.

Not mounting the directory causes tests to run on old code.

Running without interactive mode hides output.

First, build the image with tag myapp:test. Then mount current directory $(pwd) inside container and run it interactively with -it to see test output.

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