asyncio: RuntimeError this event loop is already running

Question

This seems like a common problem, see for example: RuntimeError: This event loop is already running in python

But in my case, I'm only starting the event loop once, at least as far as I can see. Also this example follows directly the instructions here:

import asyncio

loop = asyncio.get_event_loop()

async def coroutine():
    print("hey")
    await asyncio.sleep(1)
    print("ho")
    return 1


async def main():

    tasks = []
    for i in range(2):
        tasks.append(asyncio.ensure_future(coroutine()))
    await asyncio.gather(*tasks)

results = loop.run_until_complete(main())
loop.close()

This prints an error message, and the output of the print() calls in the coroutines:

---------------------------------------------------------------------------
RuntimeError                              Traceback (most recent call last)
<ipython-input-1-f4a74fbfac46> in <module>
     16     await asyncio.gather(*tasks)
     17 
---> 18 results = loop.run_until_complete(asyncio.gather(*tasks))
     19 loop.close()

~/anaconda3/envs/keras_dev/lib/python3.6/asyncio/base_events.py in run_until_complete(self, future)
    453         future.add_done_callback(_run_until_complete_cb)
    454         try:
--> 455             self.run_forever()
    456         except:
    457             if new_task and future.done() and not future.cancelled():

~/anaconda3/envs/keras_dev/lib/python3.6/asyncio/base_events.py in run_forever(self)
    407         self._check_closed()
    408         if self.is_running():
--> 409             raise RuntimeError('This event loop is already running')
    410         if events._get_running_loop() is not None:
    411             raise RuntimeError(

RuntimeError: This event loop is already running
hey
hey
ho
ho

And the results variable stays undefined.

How can I spin up a list of coroutines and gather their outputs correctly ?

Answer 1

I also came across this problem after doing some upgrades. It turns out that the tornado package is most likely the culprit. If you have tornado>=5.0 then running event loops in a notebook causes conflicts. There's a detailed discussion here but the solution, for now, is just to downgrade with pip install tornado==4.5.3.