Re: Using a background thread with asyncio/futures with flask

On 2024-03-20 10:22 AM, Thomas Nyberg via Python-list wrote:

 Hello,
 I have a simple (and not working) example of what I'm trying to do. This is a simplified version of what I'm trying to achieve (obviously the background workers and finalizer functions will do more later):
 `app.py`
 ```
import asyncio
import threading
import time
from queue import Queue
 from flask import Flask
 in_queue = Queue()
out_queue = Queue()
  def worker():
     print("worker started running")
     while True:
         future = in_queue.get()
         print(f"worker got future: {future}")
         time.sleep(5)
         print("worker sleeped")
         out_queue.put(future)
  def finalizer():
     print("finalizer started running")
     while True:
         future = out_queue.get()
         print(f"finalizer got future: {future}")
         future.set_result("completed")
         print("finalizer set result")
  threading.Thread(target=worker, daemon=True).start()
threading.Thread(target=finalizer, daemon=True).start()
 app = Flask(__name__)
  @app.route("/")
async def root():
     future = asyncio.get_event_loop().create_future()
     in_queue.put(future)
     print(f"root put future: {future}")
     result = await future
     return result
  if __name__ == "__main__":
     app.run()
```
 If I start up that server, and execute `curl http://localhost:5000`, it prints out the following in the server before hanging:
 ```
$ python3 app.py
worker started running
finalizer started running
  * Serving Flask app 'app'
  * Debug mode: off
WARNING: This is a development server. Do not use it in a production deployment. Use a production WSGI server instead.
  * Running on http://127.0.0.1:5000
Press CTRL+C to quit
root put future: <Future pending>
worker got future: <Future pending cb=[Task.task_wakeup()]>
worker sleeped
finalizer got future: <Future pending cb=[Task.task_wakeup()]>
finalizer set result
```
 Judging by what's printing out, the `final result = await future` doesn't seem to be happy here.
 Maybe someone sees something obvious I'm doing wrong here? I presume I'm mixing threads and asyncio in a way I shouldn't be.
 Here's some system information (just freshly installed with pip3 install flask[async] in a virtual environment for python version 3.11.2):
 ```
$ uname -a
Linux x1carbon 6.1.0-18-amd64 #1 SMP PREEMPT_DYNAMIC Debian 6.1.76-1 (2024-02-01) x86_64 GNU/Linux
 $ python3 -V
Python 3.11.2
 $ pip3 freeze
asgiref==3.7.2
blinker==1.7.0
click==8.1.7
Flask==3.0.2
itsdangerous==2.1.2
Jinja2==3.1.3
MarkupSafe==2.1.5
Werkzeug==3.0.1
```
 Thanks for any help!
 Cheers,
Thomas

Hi Thomas
I am no expert. However, I do have something similar in my app, and it works.
I do not use 'await future', I use 'asyncio.wait_for(future)'.
HTH
Frank Millman