Can I have Python code to continue executing after I call Flask app.run?

Question

I have just started with Python, although I have been programming in other languages over the past 30 years. I wanted to keep my first application simple, so I started out with a little home automation project hosted on a Raspberry Pi.

I got my code to work fine (controlling a valve, reading a flow sensor and showing some data on a display), but when I wanted to add some web interactivity it came to a sudden halt. Most articles I have found on the subject suggest to use the Flask framework to compose dynamic web pages. I have tried, and understood, the basics of Flask, but I just can't get around the issue that Flask is blocking once I call the "app.run" function. The rest of my python code waits for Flask to return, which never happens. I.e. no more water flow measurement, valve motor steering or display updating.

So, my basic question would be: What tool should I use in order to serve a simple dynamic web page (with very low load, like 1 request / week), in parallel to my applications main tasks (GPIO/Pulse counting)? All this in the resource constrained environment of a Raspberry Pi (3). If you still suggest Flask (because it seems very close to target), how should I arrange my code to keep handling the real-world events, such as mentioned above?

(This last part might be tough answering without seeing the actual code, but maybe it's possible answering it in a "generic" way? Or pointing to existing examples that I might have missed while searching.)

Answer 1

You're on the right track with multithreading. If your monitoring code runs in a loop, you could define a function like

def monitoring_loop():
    while True:
        # do the monitoring

Then, before you call app.run(), start a thread that runs that function:

import threading
from wherever import monitoring_loop

monitoring_thread = threading.Thread(target = monitoring_loop)
monitoring_thread.start()

# app.run() and whatever else you want to do

Don't join the thread - you want it to keep running in parallel to your Flask app. If you joined it, it would block the main execution thread until it finished, which would be never, since it's running a while True loop.

To communicate between the monitoring thread and the rest of the program, you could use a queue to pass messages in a thread-safe way between them.