Demo Testing Debugging our code locally

Welcome to this step-by-step guide on building, testing, and debugging your Flask application locally within a Docker container. By the end of this tutorial, you’ll be able to:

• Containerize a Flask app using Docker
• Identify and fix common configuration typos
• Run and verify your application on a custom host port

Prerequisites

• Python Flask application (app.py)
• requirements.txt listing Flask (and any other dependencies)
• A Dockerfile to containerize your application

Project Structure

├── app.py
├── requirements.txt
└── Dockerfile

1. Initial Dockerfile

Below is our starting Dockerfile—note the typo in the CMD instruction that we’ll address later:

FROM python:3.8-slim-buster

WORKDIR /app

COPY requirements.txt requirements.txt
RUN pip3 install -r requirements.txt

COPY .

# Note: the colon after --host is incorrect here
CMD ["python3", "-m", "flask", "run", "--host:0.0.0.0"]

2. Building the Docker Image

Open your terminal and execute:

docker build -t flask-docker-demo .

You should see output similar to:

[+] Building 2.0s (10/10) FINISHED
 => [internal] load build definition from Dockerfile             0.0s
 => [internal] load .dockerignore                                0.0s
 => [internal] load metadata for docker.io/library/python:3.8-slim-buster 0.0s
 => [1/5] FROM docker.io/library/python:3.8-slim-buster@sha256:…    0.0s
 => [2/5] WORKDIR /app                                            0.0s
 => [3/5] COPY requirements.txt requirements.txt                0.0s
 => [4/5] RUN pip3 install -r requirements.txt                  0.0s
 => [5/5] COPY .                                                 0.0s
 => exporting to image                                           0.0s
 => => writing image sha256:215f34…                              0.0s
 => => naming to docker.io/flask-docker-demo                    0.0s

Verify the image exists:

docker images | grep flask-docker-demo

3. Running the Container

Try starting the container and mapping container port 5000 to host port 5000:

docker run -p 5000:5000 flask-docker-demo

Common Port Binding Error

docker: Error response from daemon: Ports are not available: listen tcp 0.0.0.0:5000: bind: address already in use.

If you see this, remap the host port (e.g., to 5001):

docker run -p 5001:5000 flask-docker-demo

Flask Option Parsing Error

Usage: python -m flask run [OPTIONS]
Try 'python -m flask run --help' for help.

Error: No such option: --host:0.0.0.0

This happens because Flask expects --host in the form --host=<address>, not --host:<address>.

4. Fixing the Dockerfile

Update the CMD line to use = instead of ::

FROM python:3.8-slim-buster

WORKDIR /app

COPY requirements.txt requirements.txt
RUN pip3 install -r requirements.txt

COPY .

# Fixed: use '=' instead of ':'
CMD ["python3", "-m", "flask", "run", "--host=0.0.0.0"]

Rebuild the image:

docker build -t flask-docker-demo .

5. Running Successfully

Start the container on host port 5001:

docker run -p 5001:5000 flask-docker-demo

You should see:

* Debug mode: off
WARNING: This is a development server. Do not use it in a production deployment.
* Running on all addresses (0.0.0.0)
* Running on http://127.0.0.1:5000
* Running on http://172.17.0.2:5000
Press CTRL+C to quit

Open your browser at http://localhost:5001 to confirm your Flask app is live.

6. Command Reference

Next Steps

1. Commit your changes and push to GitHub.
2. Integrate with a CI/CD pipeline for automated builds.
3. Deploy to a cloud platform knowing your container behaves the same as it does locally.

Links and References

• Flask Documentation
• Docker CLI Reference
• GitHub Actions: Continuous Integration