Writing to UART via StreamWriter from Websocket results in 'NoneType object isn't iterable'

[JohannesNeu]

Hi,

I try to forward data from a websocket to UART via Microdot and Asyncio StreamWriter.
The following coroutine is used:

async def forward_to_uart(ws):
    swriter = asyncio.StreamWriter(uart, {})    
    while True:
        stringtowrite = await ws.receive()
        bytestowrite = stringtowrite.encode()
        if bytestowrite is None:
            print("It is None, proof: " + str(bytestowrite))
            pass
        else:
            print("It is not None, but: " + str(bytestowrite))
            await swriter.write(bytestowrite)
            await swriter.drain()
        await asyncio.sleep_ms(1)

This is how I start the coroutine:

@app.route('/echo')
@with_websocket
async def echo(request, ws):
    asyncio.create_task(forwarded_from_uart(ws))
    asyncio.create_task(forward_to_uart(ws))    
    while True:
        await asyncio.sleep_ms(1000)

async def main():
    await app.run(debug=True)

asyncio.run(main())

As you can see, I have tried to filter out the Nones and make sure only bytes are forwarded. Yet, nonetheless, I am getting this error message:

Starting async server on 0.0.0.0:5000...
GET /xterm 200
GET /node_modules/xterm-addon-attach/lib/xterm-addon-attach.js 200
GET /node_modules/xterm-addon-fit/lib/xterm-addon-fit.js 200
GET /node_modules/xterm/css/xterm.css 200
GET /node_modules/xterm/lib/xterm.js 200
It is not None, but: b'\x1b[A'
Task exception wasn't retrieved
future: <Task> coro= <generator object 'forward_to_uart' at 3ffddd20>
Traceback (most recent call last):
  File "asyncio/core.py", line 1, in run_until_complete
  File "<stdin>", line 30, in forward_to_uart
TypeError: 'NoneType' object isn't iterable

Where am I missing the NoneTypes?

[skyguy]

I think the problem is here:
await swriter.write(bytestowrite)
write() isn't a coroutine, so doesn't need await.

-Kevin

[JohannesNeu]

Hi @skyguy ,
that was exactly the problem! Thank you very much for the advice. I would not have been able to figure this out on my own.
Looking at the asyncio.sleep_ms functions: I chose the values at random and they seem to work, but I do not know how to get to "proper" values. Are there rules of thumb? Should I put the longer sleep in the main() function?

[skyguy]

A lot of the time, you don't need to call sleep() at all in an asyncio-based program - your program will wait for input somewhere and sit idle until the input arrives. In your code above, ws.receive() is where the program would automatically pause, so you could change the sleep to `await asyncio.sleep_ms(0)` just to give other tasks a chance to run. In programs which do need a delay (e.g. they cause the processor to run at 100%) the most common solution I've seen is to use `sleep(0.1)` - the 100ms delay isn't noticeable for humans who have just clicked a button, but is long enough that the CPU effectively sits 'idle' most of the time. Having said that, I'm no expert! Hopefully if I'm wrong someone who knows better will come along and educate us both.

…

-Kevin

[JohannesNeu]

Thank you for the explanation. That really helped.

[sophiedegran]

You can also have a look to https://github.com/peterhinch/micropython-async/blob/master/v3
They are lots of examples with asyncio