Flask runs on shared hosting with no tricks: it speaks WSGI, and that is what the Passenger behind “Setup Python App” expects. FastAPI is different: it speaks ASGI, and Passenger does not speak that directly. You can make it work with an adapter, but you lose what makes it interesting (long connections, websockets). To make real use of it, the place is a VPS. This article shows both paths. Creating the application and the virtual environment is covered in Python in cPanel: virtual environments and pip.
Which one for where
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On shared hosting (cPanel) |
On a VPS |
| Flask |
Yes, through “Setup Python App” (WSGI). |
Yes, with gunicorn behind a proxy. |
| FastAPI |
Only with an ASGI-to-WSGI adapter. No websockets and no streamed responses. |
Yes, with uvicorn behind a proxy: it is the intended design. |
| Always-on processes (queues, tasks) |
No: the application only lives while there are requests. |
Yes, with systemd, Docker or another manager. |
| Who installs and maintains |
You, in your account. |
You, on the unmanaged VPS. |
Flask in cPanel
| 1 |
Create the application under “Setup Python App” with a 3.x version, the root and the address you want. Leave the startup file as passenger_wsgi.py and the entry point as application.
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| 2 |
Put your code in the application root. For example, an app.py with app = Flask(__name__).
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| 3 |
Connect Passenger to Flask: in passenger_wsgi.py write just from app import app as applicationThe name application is what Passenger looks for.
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| 4 |
Install the libraries with a requirements.txt and “Run Pip Install”, and restart.
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| 5 |
Open the address. If it errors, the Passenger log (the Passenger log file field) says what failed.
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FastAPI
In cPanel, with an adapter. Install fastapi and a2wsgi (an ASGI-to-WSGI adapter), and in passenger_wsgi.py:from a2wsgi import ASGIMiddleware
from main import app
application = ASGIMiddleware(app)It works for an API of simple requests and responses. Do not count on websockets.
On a VPS. Inside the project’s virtual environment, install fastapi and uvicorn and start it: uvicorn main:app --host 127.0.0.1 --port 8000The main:app means “the file main.py, the variable app”. Listening only on 127.0.0.1, what shows it to the world is nginx with HTTPS: nginx as a reverse proxy. To keep it up after you close the session, use systemd or Docker. The same goes for Flask on a VPS, swapping uvicorn for gunicorn -b 127.0.0.1:8000 app:app.
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Never leave debug mode on in production. In Flask, debug=True (or FLASK_DEBUG) opens a console in the browser that lets anyone who reaches an error page run code on the server. And do not use the development server (flask run) or uvicorn --reload as your public server.
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The application opens locally but not on the server? Tell us the domain and the last line of the Passenger log.
Open a support ticket
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