Docker containerization in Flask - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What happens when you run this Dockerfile for a Flask app?

Consider this Dockerfile for a simple Flask app. What will be the output when you run the container and access the root URL?

Flask

FROM python:3.12-slim
WORKDIR /app
COPY requirements.txt ./
RUN pip install -r requirements.txt
COPY . ./
ENV FLASK_APP=app.py
CMD ["flask", "run", "--host=0.0.0.0"]

A The Flask app starts and is accessible on port 5000 inside the container.

B The container fails to start because the CMD syntax is incorrect.

C The Flask app starts but is only accessible on localhost inside the container, not from outside.

D The container starts but Flask raises an error because FLASK_APP is not set.

💡 Hint

Think about how Flask's default port and host settings work inside Docker.

The Dockerfile sets FLASK_APP and runs Flask with --host=0.0.0.0, making the app listen on all interfaces inside the container, typically port 5000.

📝 Syntax

intermediate

Which Dockerfile command correctly copies only Python files to /app?

You want to copy only .py files from your project root to /app in the container. Which COPY command is correct?

A COPY /root/*.py /app/

B COPY /app/*.py /app/

C COPY ./*.py /app/

D COPY *.py /app/

💡 Hint

Remember COPY paths are relative to the build context.

COPY *.py /app/ copies all Python files from the build context root to /app in the container.

🔧 Debug

advanced

Why does this Flask app inside Docker fail to connect to the database?

You have a Flask app in Docker that connects to a database at 'localhost:5432'. The app fails to connect. Why?

Flask

DATABASE_URL = 'postgresql://user:pass@localhost:5432/dbname'
app.config['SQLALCHEMY_DATABASE_URI'] = DATABASE_URL

A Because 'localhost' inside the container points to the container itself, not the host machine.

B Because the database URL is missing the port number.

C Because Flask cannot connect to databases inside Docker containers.

D Because the database URL must use '127.0.0.1' instead of 'localhost'.

💡 Hint

Think about network isolation in Docker containers.

'localhost' inside a container refers to the container itself, so it cannot reach services running on the host machine.

❓ state_output

advanced

What is the effect of this Docker Compose service configuration on Flask app state?

Given this docker-compose.yml snippet, what happens to the Flask app's code changes on the host during container runtime?

Flask

services:
  web:
    build: .
    volumes:
      - .:/app
    ports:
      - "5000:5000"

A The container crashes because volumes cannot be used with Flask apps.

B Code changes on the host immediately reflect inside the container without rebuilding.

C Code changes require rebuilding the image to take effect inside the container.

D The container ignores the volume and uses the image code only.

💡 Hint

Volumes link host files to container files in real time.

The volume mounts the current directory into /app inside the container, so changes on the host appear immediately inside the container.

🧠 Conceptual

expert

Why use multi-stage builds in Docker for Flask apps?

What is the main benefit of using multi-stage Docker builds when containerizing a Flask application?

A To allow running multiple Flask apps in one container.

B To enable Flask apps to automatically restart on code changes.

C To reduce the final image size by separating build dependencies from runtime environment.

D To make the container run faster by using multiple CPU cores.

💡 Hint

Think about what stays in the final image after building.

Multi-stage builds let you install build tools and dependencies in one stage, then copy only the needed files to a smaller final image.