arduino yun uhttpd flask setup

Question

I'm trying to set up python and flask on the arduino yun. I've managed to run python files via the /etc/config/uhttpd configuration file:

...
list interpreter    ".py=/usr/bin/python"
...

The default path for the website's root is: /www in which I've placed a soft link (apps) to the sd card. So now I can run python programs: http://[ip arduino]/apps/helloworld.py

And when I make my first helloflask.py program and run that via python helloflask.py I can see the result at: http://[ip arduino]:5000

But now I want to configure the uhttpd mini webserver (which is capable to exchange information via CGI) to use the flask setup. The URI: http://flask.pocoo.org/docs/deploying/cgi/#server-setup shows some instructions... but I just don't get it. I've made a directory ../apps/uno in which I've placed a __init__.py file with the following content:

from flask import Flask
app = Flask(__name__)

@app.route("/")
def hello():
    return "He Flask!"   

In the apps dir I've put a file: cgi.py with this content:

from wsgiref.handlers import CGIHandler
from uno import app

CGIHandler().run(app)

Now I when I browse: http://[ip arduino]/cgi.py get a server error occured, contact the administrator (I think this is the CGI interface from uhttpd).

I just don't grasp the CGI configuration for Flask/uhttpd

Answer 1

I looked into this too and got a little further, I was able to setup a simple hello world but once I tried to do something non-trivial I ran into a big issue that uhttpd doesn't support URL rewriting/aliasing. This means your flask app can only be served at the URL of its .py file instead of at a root like http:// (arduino IP) /flaskapp/. None of the routes inside the app will be visible and makes the whole thing unusable.

However, instead of trying to force flask into uhttpd I had great success running the built in server that flask provides. Take a look at this guide I wrote up that uses flask to serve data from a Yun: https://learn.adafruit.com/smart-measuring-cup/overview

The thing to do is add a call to app.run when the script is run, for example make your flask app look like:

from flask import Flask
app = Flask(__name__)

@app.route("/")
def hello():
    return "Hello Flask!"

if __name__ == '__main__':
    app.run(host='0.0.0.0', debug=True, threaded=True)

Then log in to the Yun and run the script using python. Flask's built in server should start serving the app on http:// (arduino IP) :5000/. Make sure to include the host='0.0.0.0' as it's required to listen on the Yun's external network interface. You probably also want debug=True so there are better error messages (and live reloading of the server when the code changes), and I found threaded=True helps because the default server only handles one connection at a time. The Yun is a relatively slow processor so don't expect to service a lot of concurrent requests, however it's quite capable for providing a simple REST API or web application for a few users.

If you want this server to always run on bootup, edit the /etc/rc.local file to include a call to python and your script.