Docker containerization in Flask - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to specify the base image in a Dockerfile for a Flask app.

Flask

FROM [1]

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

Aubuntu:20.04

Bnode:latest

Cpython:3.12-slim

Dalpine:latest

Attempts:

3 left

💡 Hint

Common Mistakes

Choosing a Node.js or Ubuntu image which is not optimized for Python Flask apps.

The base image python:3.12-slim is a lightweight Python image suitable for Flask apps.

2fill in blank

medium

Complete the code to copy the Flask app files into the Docker image.

Flask

COPY [1] /app

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

Aapp.py

B.

CDockerfile

Drequirements.txt

Attempts:

3 left

💡 Hint

Common Mistakes

Copying only requirements.txt or app.py which misses other needed files.

Copying . copies all files from the current directory into the image's /app folder.

3fill in blank

hard

Fix the error in the Dockerfile command to install dependencies.

Flask

RUN pip install -r [1]

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

Arequirements.txt

BDockerfile

Capp.py

Dsetup.py

Attempts:

3 left

💡 Hint

Common Mistakes

Trying to install from app.py or Dockerfile which are not package lists.

The dependencies are listed in requirements.txt, so pip installs from that file.

4fill in blank

hard

Fill both blanks to set the working directory and expose the Flask app port.

Flask

WORKDIR [1]
EXPOSE [2]

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

A/app

B8080

C5000

D/usr/src/app

Attempts:

3 left

💡 Hint

Common Mistakes

Exposing the wrong port or setting the wrong working directory.

The working directory is set to /app where the app files are copied. Flask's default port is 5000, so we expose that.

5fill in blank

hard

Fill all three blanks to define the command that runs the Flask app inside the container.

Flask

CMD ["[1]", "[2]", "[3]"]

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

Apython

Bapp.py

C-m

Dflask

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'flask' directly without python or missing the module flag.

The command runs Python with the script app.py to execute the Flask app.

Practice

1. What is the main purpose of using Docker with a Flask application?

easy

A. To package the Flask app and its environment so it runs consistently anywhere

B. To write Flask application code faster

C. To replace Flask's routing system

D. To automatically generate HTML templates

Solution

1. Step 1: Understand Docker's role

   Docker packages an app with all its dependencies into a container.

2. Step 2: Connect Docker to Flask app usage

   This packaging ensures the Flask app runs the same on any machine without setup issues.

3. Final Answer:

   To package the Flask app and its environment so it runs consistently anywhere -> Option A

4. Quick Check:

   Docker packages app + environment = B [OK]

Hint: Docker ensures app runs same everywhere by packaging environment [OK]

Common Mistakes:

• Thinking Docker speeds up coding
• Confusing Docker with Flask features
• Assuming Docker changes app logic

2. Which of the following is the correct first line in a Dockerfile for a Flask app using Python 3.12?

easy

A. EXPOSE 5000

B. RUN python3.12 install flask

C. COPY app.py /app

D. FROM python:3.12-slim

Solution

1. Step 1: Identify Dockerfile base image syntax

   The Dockerfile must start with a FROM statement specifying the base image.

2. Step 2: Choose correct Python version image

   "FROM python:3.12-slim" correctly sets Python 3.12 as base.

3. Final Answer:

   FROM python:3.12-slim -> Option D

4. Quick Check:

   Dockerfile starts with FROM base image = D [OK]

Hint: Dockerfile always starts with FROM base image line [OK]

Common Mistakes:

• Using RUN before FROM
• Copying files before setting base image
• Exposing ports before base image

3. Given this Dockerfile snippet for a Flask app:

FROM python:3.12-slim
WORKDIR /app
COPY requirements.txt ./
RUN pip install -r requirements.txt
COPY . .
CMD ["python", "app.py"]

What happens when you run docker build . and then docker run?

medium

A. The app fails because requirements.txt is not copied

B. The Flask app runs inside a container using Python 3.12 with dependencies installed

C. The container runs but does not start the Flask app

D. The build command installs Flask globally on the host machine

Solution

1. Step 1: Analyze Dockerfile commands

   It sets Python 3.12 base, copies requirements.txt, installs dependencies, copies app files, and runs app.py.

2. Step 2: Understand build and run effects

   docker build creates image with dependencies; docker run starts container running Flask app.

3. Final Answer:

   The Flask app runs inside a container using Python 3.12 with dependencies installed -> Option B

4. Quick Check:

   Build installs deps, run starts app = A [OK]

Hint: COPY then RUN pip install ensures dependencies inside container [OK]

Common Mistakes:

• Thinking dependencies install on host
• Assuming app.py is not copied
• Believing container runs without CMD

4. You wrote this Dockerfile for your Flask app:

FROM python:3.12-slim
COPY app.py /app/
CMD ["python", "app.py"]

When you run the container, the app crashes with "ModuleNotFoundError: flask". What is the problem?

medium

A. Flask is not installed inside the container

B. app.py is not copied to the container

C. CMD syntax is incorrect

D. Python version 3.12-slim does not support Flask

Solution

1. Step 1: Check Dockerfile for dependency installation

   No RUN command installs Flask or dependencies inside container.

2. Step 2: Understand error cause

   ModuleNotFoundError means Flask package missing in container environment.

3. Final Answer:

   Flask is not installed inside the container -> Option A

4. Quick Check:

   Missing Flask install causes ModuleNotFoundError = C [OK]

Hint: Always RUN pip install for dependencies inside Dockerfile [OK]

Common Mistakes:

• Assuming Flask is preinstalled in base image
• Thinking app.py copy is missing
• Believing CMD syntax causes import errors

5. You want to containerize a Flask app that reads environment variables for configuration. Which Dockerfile addition ensures environment variables set at runtime are accessible inside the Flask app?

hard

A. Hardcode variables inside app.py before building image

B. Use ENV in Dockerfile to set variables permanently

C. Pass variables with docker run -e VAR=value and access via os.environ in Flask

D. Add EXPOSE for environment variables

Solution

1. Step 1: Understand environment variable usage

   Flask app reads variables at runtime from OS environment.

2. Step 2: Identify Docker method to pass runtime variables

   Using docker run -e VAR=value passes variables dynamically to container.

3. Final Answer:

   Pass variables with docker run -e VAR=value and access via os.environ in Flask -> Option C

4. Quick Check:

   Runtime env vars via docker run -e = A [OK]

Hint: Use docker run -e to pass env vars dynamically to container [OK]

Common Mistakes:

• Confusing ENV for runtime variable passing
• Hardcoding vars loses flexibility
• Thinking EXPOSE relates to env vars