[FIXED] After first run ,Jupyter notebook with python 3.6.1, using asyncio basic example gives: RuntimeError: Event loop is closed

Issue

In Jupyter Notebook (python 3.6.1) I went to run the basic python docs Hello_World in (18.5.3.1.1. Example: Hello World coroutine) and noticed that it was giving me a RuntimeError. After trying a long time to find the problem with the program(my understanding is that the docs may not be totally up to date), I finally noticed that it only does this after the second run and tested in a restarted Kernel. I've since then copied the same small python program in two successive cells(In 1 and 2) and found that it gives the error on the second not the first and gives the error to both there after. This repeats this after restarting the Kernel.

import asyncio

def hello_world(loop):
    print('Hello World')
    loop.stop()

loop = asyncio.get_event_loop()

# Schedule a call to hello_world()
loop.call_soon(hello_world, loop)

# Blocking call interrupted by loop.stop()
loop.run_forever()
loop.close() 

The traceback:

RuntimeError                  Traceback (most recent call last)
<ipython-input-2-0930271bd896> in <module>()
  6 loop = asyncio.get_event_loop()
  7 # Blocking call which returns when the hello_world() coroutine
----> 8 loop.run_until_complete(hello_world())
  9 loop.close()

/home/pontiac/anaconda3/lib/python3.6/asyncio/base_events.py in run_until_complete(self, future)
441         Return the Future's result, or raise its exception.
442         """
--> 443         self._check_closed()
444 
445         new_task = not futures.isfuture(future)

/home/pontiac/anaconda3/lib/python3.6/asyncio/base_events.py in     _check_closed(self)
355     def _check_closed(self):
356         if self._closed:
--> 357             raise RuntimeError('Event loop is closed')
358 
359     def _asyncgen_finalizer_hook(self, agen):

RuntimeError: Event loop is closed

I don't get this error when running a file in the interpreter with all the Debug settings set. I am running this Notebook in my recently reinstalled Anaconda set up which only has the 3.6.1 python version installed.

Solution

the issue is that loop.close() makes the loop unavailable for future use. That is, you can never use a loop again after calling close. The loop stays around as an object, but almost all methods on th eloop will raise an exception once the loop is closed. However, asyncio.get_event_loop() returns the same loop if you call it more than once. You often want this, so that multiple parts of an application get the same event loop. However if you plan on closing a loop, you are better off calling asyncio.new_event_loop rather than asyncio.get_event_loop. That will give you a fresh event loop. If you call new_event_loop rather than get_event_loop, you're responsible for making sure that the right loop gets used in all parts of the application that run in this thread. If you want to be able to run multiple times to test you could do something like:

loop = asyncio.new_event_loop()
asyncio.set_event_loop(loop)

After that, you'll find that asyncio.get_event_loop returns the same thing as loop. So if you do that near the top of your program, you will have a new fresh event loop each run of the code.

Answered By - Sam Hartman