In the documentation for Sample App for Python, under Making Changes to Your App (https://docs.digitalocean.com/products/app-platform/quickstart/sample-apps/python/#making-changes-to-your-app), the first list item says:
Edit index.php and replace “Hello!” on line 12 with a different greeting
However, I believe the file that is meant to refer to is
server.py
from this repository: https://github.com/digitalocean/sample-python/blob/main/server.py