Start a Flask app and the terminal prints a familiar line: "WARNING: This is a development server. Do not use it in a production deployment." Yet plenty of desktop apps bundle that same local Flask server as their actual runtime and keep it running on the user's machine for the life of the session. That looks like ignoring the warning outright, but the underlying assumptions have actually changed. This article works through what has to change for that warning to become safe to set aside — and what you still have to handle yourself, or it turns into a real bug.
Note: WSGI (Web Server Gateway Interface) is the standard interface between a Python web application and the server that runs it. Flask itself builds the WSGI application; the part that actually accepts and serves HTTP requests is a separate, swappable component. By default, development uses a lightweight built-in server (Werkzeug), while production deployments normally swap in a dedicated production WSGI server such as Gunicorn.
The warning is about an unpredictable crowd of clients
What that warning is really about is a public-facing web service: handling concurrent traffic from an unknown number of clients, minimal built-in hardening, and no multi-worker process model for load distribution. In short, "not strong enough to serve the open internet."
