Using a background thread with asyncio/futures with flask

Fri Mar 22 03:27:28 EDT 2024

Hey,

As far as I know (might be old news) flask does not support asyncio.

You would have to use a different framework, like e.g. FastAPI or similar. Maybe someone has already written "flask with asyncio" but I don't know about that.

Cheers

Lars

Lars Liedtke
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Am 20.03.24 um 09:22 schrieb Thomas Nyberg via Python-list:

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