Using a background thread with asyncio/futures with flask

[Thomas Nyberg]

Hi,

Yeah so flask does support async (when installed with `pip3 install 
flask[async]), but you are making a good point that flask in this case 
is a distraction. Here's an example using just the standard library that 
exhibits the same issue:

`app.py`
```
import asyncio
import threading
import time
from queue import Queue


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).start()
threading.Thread(target=finalizer).start()


async def main():
     future = asyncio.get_event_loop().create_future()
     in_queue.put(future)
     print(f"main put future: {future}")
     result = await future
     print(result)


if __name__ == "__main__":
     loop = asyncio.get_event_loop()
     loop.run_until_complete(main())
```

If I run that I see the following printed out (after which is just hangs):

```
$ python3 app.py
worker started running
finalizer started running
main put future: <Future pending>
worker got future: <Future pending>
worker sleeped
finalizer got future: <Future pending cb=[Task.task_wakeup()]>
finalizer set result
```

I believe async uses a cooperative multitasking setup under the hood, so 
I presume the way I'm doing this threading just isn't playing well with 
that (and presumably some csp yield isn't happening somewhere). Anyway 
at this point I feel like the easiest approach is to just throw away 
threads entirely and learn how to do all I want fully in the brave new 
async world, but I'm still curious why this is failing and how to make 
this sort of setup work since it points to my not understanding the 
basic implementation/semantics of async in python.

Thanks for any help!

/Thomas

On 3/22/24 08:27, Lars Liedtke via Python-list wrote:
> 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
> Lead Developer
> 
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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