python 3.x - sys:1: RuntimeWarning: coroutine was never awaited

Question

I am trying to write a request handler to help me send request in async mode. It prompt when I close the python terminal with Ctrl+D or exit()

It shows sys:1: RuntimeWarning: coroutine was never awaited

import asyncio
import urllib.request
import json 

class RequestHandler:
    def SendPostRequest(method="post",url=None, JsonFormatData={}):
        # Encode JSON
        data =json.dumps(JsonFormatData).encode('utf8')
        # Config Request Header
        req = urllib.request.Request(url)
        req.add_header('Content-Type', 'application/json')      
        # Send request and wait the response
        response = urllib.request.urlopen(req,data=data)    
        return response 

    async def AsyncSend(method="post",url=None, JsonFormatData=None):
        if method == "post":
            loop = asyncio.get_event_loop()
            task = loop.create_task(SendPostRequest(method="post",url=url,JsonFormatData=JsonFormatData))

###################################
# Example
##### In main python terminal, i run like this:
# from RequestHandler import * 
# RequestHandler.AsyncSend(method="post",url="xxxxxx", JsonFormatData={'key':'value'} )

When I Click Ctrl+D , it prompted

sys:1: RuntimeWarning: coroutine 'RequestHandler.AsyncSend' was never awaited

Is that I shall ignore it? I don't wanna call await, as i don't care if the process is success or not.

In this link "https://xinhuang.github.io/posts/2017-07-31-common-mistakes-using-python3-asyncio.html", it said that "To execute an asynchronous task without await, use loop.create_task() with loop.run_until_complete()", is that wrong then?

Answer 1

I think you're confusing JS async API with Python's. In Python, when you call a coroutine function it returns a coroutine (similar to an armed generator) but doesn't schedule it in the event loop. (i.e. doesn't run/consume it)

You have two options:

1) You can await it via await or the older yield from.

2) You can asyncio.create_task(coroutine_function()). This is the equivalent of calling a promise in JS without giving it a handler or awaiting it.

The warning you're seeing is telling you that the coroutine didn't run. it was only created, but not consumed.

As to your code, there are two errors. First urllib is a blocking library, you can't create a task from it, neither can it be ran asynchronously, take a look at aiohttp.ClientSession instead.

Second, the warning you're seeing is probably caused by you calling AsyncSend synchronously (without awaiting it). Again, in JS this would probably be fine, as everything in JS is async. In Python, you should use one of the two main methods I mentioned above.

If you insist on using a blocking library, you can run it on a different thread so that you don't block the event loop. As Cloudomation mentioned, to do this. you should use asyncio.run_in_executor(None, lambda: your_urllib_function())