Running a Flask Web App within a container – lesson learnt! 🫙

Over the years I’ve created a few web apps in Python using the Flask Framework, a good (or not so good) example can be found here.

I typically host these within an Azure Web App, recently I was experimenting with container instances within OCI (Oracle Cloud Infrastructure), I set about attempting to port one of my web apps to run within a container, I ran into a small issue that I thought I’d document here (mainly for my future self).

When deploying a Python web app to an Azure Web App that uses Flask, I use the following code to run the web app (at the bottom of the Pythion application.py file – which contains the code for the Flask web app):

if __name__ == "__main__":
    app.run()

This works like a charm locally (running the web app on http://localhost:5000) and when published to an Azure Web App, this runs on port 443 (https), for example https://(webappname).azurewebsites.net.

When running this app within a container, it failed miserably and the site didn’t render ☹️. Looking at the logs within OCI this was because the container was listening on port 5000 (as it would typically do when running locally).

It turned out that I needed to update the application.py file and configure the port to listen on and override the default of port 5000 (as below – I used port 80/http to keep this simple).

if __name__ == "__main__":
    app.run(port=80,host='0.0.0.0')

This did the trick and my web app worked correctly! It looks like Azure (Gunicorn) does some magic under the hood and override’s the default behaviour of listening on port 5000.

Whilst I ran into this issue using OCI, it would be the same if I was running Docker locally, Azure, GCP or AWS – it was an issue with me, rather than OCI 🤦‍♂️.