> ## Documentation Index
> Fetch the complete documentation index at: https://notes.kodekloud.com/llms.txt
> Use this file to discover all available pages before exploring further.
# Demo Testing Debugging our code locally
> This article provides a guide for building, testing, and debugging a Flask application within a Docker container.
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](https://docs.docker.com/get-docker/) 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
```text theme={null}
├── 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:
```dockerfile theme={null}
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:
```bash theme={null}
docker build -t flask-docker-demo .
```
You should see output similar to:
```bash theme={null}
[+] 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:
```bash theme={null}
docker images | grep flask-docker-demo
```
## 3. Running the Container
Try starting the container and mapping container port 5000 to host port 5000:
```bash theme={null}
docker run -p 5000:5000 flask-docker-demo
```
### Common Port Binding Error
```bash theme={null}
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):
```bash theme={null}
docker run -p 5001:5000 flask-docker-demo
```
### Flask Option Parsing Error
```bash theme={null}
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=`, not `--host:`.
## 4. Fixing the Dockerfile
Update the `CMD` line to use `=` instead of `:`:
```dockerfile theme={null}
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:
```bash theme={null}
docker build -t flask-docker-demo .
```
## 5. Running Successfully
Start the container on host port 5001:
```bash theme={null}
docker run -p 5001:5000 flask-docker-demo
```
You should see:
```bash theme={null}
* 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](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](https://gunicorn.org/) or [uWSGI](https://uwsgi-docs.readthedocs.io/).
## 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
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](https://flask.palletsprojects.com/)
* [Docker CLI Reference](https://docs.docker.com/engine/reference/commandline/docker/)
* [GitHub Actions: Continuous Integration](https://docs.github.com/en/actions)