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

Ensure you have Docker installed and Python 3.8+ on your local machine before you begin.

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.

This built-in Flask server is for development only. For production deployments, use a WSGI server like Gunicorn or uWSGI.

6. Command Reference

Command	Description
`docker build -t flask-docker-demo .`	Build the Docker image for the Flask app
`docker images`	List all local Docker images
`docker run -p HOST:5000 flask-...`	Run container, mapping host port to container
`docker run -p 5001:5000 flask-docker-demo`	Remap host port if default is unavailable

Next Steps

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

Introduction

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Conclusion

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Demo Testing Debugging our code locally

Prerequisites

Project Structure

1. Initial Dockerfile

2. Building the Docker Image

3. Running the Container

Common Port Binding Error

Flask Option Parsing Error

4. Fixing the Dockerfile

5. Running Successfully

6. Command Reference

Next Steps

Links and References

Watch Video

​Prerequisites

​Project Structure

​1. Initial Dockerfile

​2. Building the Docker Image

​3. Running the Container

​Common Port Binding Error

​Flask Option Parsing Error

​4. Fixing the Dockerfile

​5. Running Successfully

​6. Command Reference

​Next Steps

​Links and References

Watch Video

Prerequisites

Project Structure

1. Initial Dockerfile

2. Building the Docker Image

3. Running the Container

Common Port Binding Error

Flask Option Parsing Error

4. Fixing the Dockerfile

5. Running Successfully

6. Command Reference

Next Steps

Links and References