“RuntimeError: This event loop is already running”; debugging aiohttp, asyncio and IDE “spyder3” in python 3.6.5

Question

I\'m struggling to understand why I am getting the \"RuntimeError: This event loop is already running\" runtime error. I have tried to run snippets of code from \"https://aiohtt

Answer 1

Looks to me Spyder runs its own event loop. You cannot run two event loops in a single thread.

asyncio.run(coro, *, debug=False)

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

This is what worked for me. I start my own loop if there is not other running loop:

import asyncio

async def say_after(delay, what):
    await asyncio.sleep(delay)
    print(what)

async def main():
    await say_after(2, 'done')

await say_after(1, 'ahoy')    

loop = asyncio.get_event_loop()
print(loop) # <_WindowsSelectorEventLoop running=True closed=False debug=False>
if loop.is_running() == False:
    asyncio.run(main())
else:
    await main()

Answer 2

Perhaps I was lucky, but I downgraded Tornado. See "Can't invoke asyncio event_loop after tornado 5.0 update"

Answer 3

I have the same issue with Spyder, The only solution that worked for me was to use nest_asyncio

install the nest_asyncio by using the command

pip install nest_asyncio

Add the below lines in your file

import nest_asyncio
nest_asyncio.apply()

And the issue must be fixed.

---

From the docs's

By design asyncio does not allow its event loop to be nested. This presents a practical problem: When in an environment where the event loop is already running it’s impossible to run tasks and wait for the result. Trying to do so will give the error “RuntimeError: This event loop is already running”.

The issue pops up in various environments, such as web servers, GUI applications and in Jupyter notebooks.

This module patches asyncio to allow nested use of asyncio.run and loop.run_until_complete.

Answer 4

The issue appears to be related to the IDE used (Spyder3). I tried running the code with PyCharm community edition last night. The code ran with no issues.

I have submitted a bug to Spyder3.