RuntimeError: asyncio.run() cannot be called from a running event loop

Question

I am trying to learn about asyncio for a websockets client. Every piece of code I try gets the following error:

RuntimeError: asyncio.run() cannot be called from a running event loop

I have tried the most simple code and it always gives that RuntimeError. I tried installing the full anaconda distribution again, etc, and can´t find what the problem might be.

I am using Spyder 3.3.3 with Python 3.7.3

An example of code that should work:

import asyncio

async def main():
    print('hello')
    await asyncio.sleep(1)
    print('world')

asyncio.run(main())

Error message:

File "C:\Users\jmart\Anaconda3\lib\site-packages\spyder_kernels\customize\spydercustomize.py", line 786, in runfile
  execfile(filename, namespace)
File "C:\Users\jmart\Anaconda3\lib\site-packages\spyder_kernels\customize\spydercustomize.py", line 110, in execfile
  exec(compile(f.read(), filename, 'exec'), namespace)
File "C:/Users/jmart/Documents/asynk2.py", line 8, in <module>
  asyncio.run(main())
File "C:\Users\jmart\Anaconda3\lib\asyncio\runners.py", line 34, in run
  "asyncio.run() cannot be called from a running event loop")
RuntimeError: asyncio.run() cannot be called from a running event loop

Answer 1

It's a known problem related to IPython.

One way as you already found is to use nest_asyncio :

import nest_asyncio
nest_asyncio.apply()

The other one is to install older version of tornado :

pip3 install tornado==4.5.3

Answer 2

Problem origin

Spyder runs its own event loop

print(asyncio.get_running_loop().is_running()) 
Returns: True

but only one is allowed per thread

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

That is why, when we try to create a new event loop with
asyncio.run(main()) it gives us an error:
RuntimeError: asyncio.run() cannot be called from a running event loop

Solution

Except what already proposed with nest_asyncio and tornado I came up with

1. Attach to an existing Spyder thread event loop by creating a new task

import asyncio

async def main():
    print('Hello world!')

asyncio.create_task(main())

2. Create new thread (by executing in external terminal) which allows to run our own event loop.
   Top menu Run -> Run configuration per file... -> Execute in the external system terminal

Now the code runs in new terminal and works

import asyncio

async def main():
    print('Hello world!')
asyncio.run(main())