"asyncio.run() cannot be called from a running event loop" when using Jupyter Notebook

Question

I would like to use asyncio to get webpage html.

I run the following code in jupyter notebook:

import aiofiles
import aiohttp
from aiohttp import ClientSession

async def get_info(url, session):
    resp = await session.request(method="GET", url=url)
    resp.raise_for_status()
    html = await resp.text(encoding='GB18030')
    with open('test_asyncio.html', 'w', encoding='utf-8-sig') as f:
        f.write(html)
    return html
    
async def main(urls):
    async with ClientSession() as session:
        tasks = [get_info(url, session) for url in urls]
        return await asyncio.gather(*tasks)

if __name__ == "__main__":
    url = ['http://huanyuntianxiazh.fang.com/house/1010123799/housedetail.htm', 'http://zhaoshangyonghefu010.fang.com/house/1010126863/housedetail.htm']
    result = asyncio.run(main(url))

However, it returns RuntimeError: asyncio.run() cannot be called from a running event loop

What is the problem?

How to solve it?

Answer 1

The asyncio.run() documentation says:

This function cannot be called when another asyncio event loop is running in the same thread.

In your case, jupyter (IPython ≥ 7.0) is already running an event loop:

You can now use async/await at the top level in the IPython terminal and in the notebook, it should — in most of the cases — “just work”. Update IPython to version 7+, IPykernel to version 5+, and you’re off to the races.

Therefore you don't need to start the event loop yourself and can instead call await main(url) directly, even if your code lies outside any asynchronous function.

Jupyter (IPython ≥ 7.0)

async def main():
    print(1)
    
await main()

Python ≥ 3.7 and IPython < 7.0

import asyncio

async def main():
    print(1)
    
asyncio.run(main())

In your code that would give:

url = ['url1', 'url2']
result = await main(url)

for text in result:
    pass # text contains your html (text) response

Caution

There is a slight difference on how Jupyter uses the loop compared to IPython.

Answer 2

To add to cglacet's answer - if one wants to detect whether a loop is running and adjust automatically (ie run main() on the existing loop, otherwise asyncio.run()), here is a snippet that may prove useful:

# async def main():
#     ...

try:
    loop = asyncio.get_running_loop()
except RuntimeError:  # 'RuntimeError: There is no current event loop...'
    loop = None

if loop and loop.is_running():
    print('Async event loop already running. Adding coroutine to the event loop.')
    tsk = loop.create_task(main())
    # ^-- https://docs.python.org/3/library/asyncio-task.html#task-object
    # Optionally, a callback function can be executed when the coroutine completes
    tsk.add_done_callback(
        lambda t: print(f'Task done with result={t.result()}  << return val of main()'))
else:
    print('Starting new event loop')
    result = asyncio.run(main())

Answer 3

Just use this:

https://github.com/erdewit/nest_asyncio

import nest_asyncio
nest_asyncio.apply()

Answer 4

Combining the methods from Pankaj Sharma and Jean Monet, I wrote the following snippet that acts as asyncio.run (with slightly different syntax), but also works within a Jupyter notebook.

class RunThread(threading.Thread):
    def __init__(self, func, args, kwargs):
        self.func = func
        self.args = args
        self.kwargs = kwargs
        self.result = None
        super().__init__()

    def run(self):
        self.result = asyncio.run(self.func(*self.args, **self.kwargs))

def run_async(func, *args, **kwargs):
    try:
        loop = asyncio.get_running_loop()
    except RuntimeError:
        loop = None
    if loop and loop.is_running():
        thread = RunThread(func, args, kwargs)
        thread.start()
        thread.join()
        return thread.result
    else:
        return asyncio.run(func(*args, **kwargs))

Usage:

async def test(name):
    await asyncio.sleep(5)
    return f"hello {name}"

run_async(test, "user")  # blocks for 5 seconds and returns "hello user"

Answer 5

To add to existing answers, a somewhat shorter version without external libraries, that allows running inside and outside of jupyter and allows fetching the return value:

try:
    asyncio.get_running_loop()
    # we need to create a separate thread so we can block before returning
    with ThreadPoolExecutor(1) as pool:
        result = pool.submit(lambda: asyncio.run(myfunc())).result()
except RuntimeError:
    # no event loop running
    result = asyncio.run(myfunc())

Answer 6

I found the unsync package useful for writing code that behaves the same way in a Python script and the Jupyter REPL.

import asyncio
from unsync import unsync


@unsync
async def demo_async_fn():
    await asyncio.sleep(0.1)
    return "done!"

print(demo_async_fn().result())

Answer 7

Slight simplification of the solution by Mark:

import threading

class RunThread(threading.Thread):
    def __init__(self, coro):
        self.coro = coro
        self.result = None
        super().__init__()

    def run(self):
        self.result = asyncio.run(self.coro)


def run_async(coro):
    try:
        loop = asyncio.get_running_loop()
    except RuntimeError:
        loop = None
    if loop and loop.is_running():
        thread = RunThread(coro)
        thread.start()
        thread.join()
        return thread.result
    else:
        return asyncio.run(coro)

Use run_async() like async.run(), i.e., run_async(test("user")).

Answer 8

As cglacet mentioned that documentation says

This function cannot be called when another asyncio event loop is running in the same thread.

You can use another thread i.e -

class ResolveThread(threading.Thread):
            def __init__(self,result1,fun,url):
                self.result1= result1
                self.fun = fun
                self.url = url
                threading.Thread.__init__(self)
            def run(self):
                result1[0] = asyncio.run(self.fun(self.url))


result1 = [None]
sp = ResolveThread(result1)
sp.start()
sp.join() # connect main thread
result = result1[0]