Can't create new event loop after calling loop.close asyncio.get_event_loop in Python3.6.1

Question

In Python3.6.1 after calling loop.close() on a loop obtained from asyncio.get_event_loop(), can a new loop be created?

I've looked at other posts with answers for properly closing a loop, and also how to use task.cancel(), but have not been able to use any of those examples in a way that would let a new loop be created after the first was closed. I also tried explicitly setting executor and later calling executor.shutdown(wait=True), but that didn't help. I also tried 'del loop', and del a bunch of other things.

Documentation indicates that closing a an event loop is idempotent and irreversible. Does that also mean that a new loop cannot be created?

Here is some simple example code to demonstrate the issue: `



#!/usr/bin/env python3.6
'''
To demonstrate an issue, the following code was adapted from:
https://docs.python.org/3/library/asyncio-eventloop.html
'''
import asyncio

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

loop = asyncio.get_event_loop()
loop.call_soon(hello_world, loop)
loop.run_forever()
# loop.close()


'''
If the commented out loop.close() above is uncommented,
the following code will fail with:
    Traceback (most recent call last):
        File "./aquestion.py", line 28, in <module>
            loopNew.call_soon(hello_world, loopNew)
        File "/Library/Frameworks/Python.framework/Versions/3.6/lib    /python3.6/asyncio/base_events.py", line 573, in call_soon
        self._check_closed()
        File "/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/asyncio/base_events.py", line 357, in _check_closed
            raise RuntimeError('Event loop is closed')
    RuntimeError: Event loop is closed

'''
loopNew = asyncio.get_event_loop()
loopNew.call_soon(hello_world, loopNew)
loopNew.run_forever()

`

Any attempts to answer my question would be appreciated.

Alternatively, would it be bad form to create an event loop, use it for a variety of purposes, and then just close that loop when the long running program is about to exit? This just seems wrong.

dirn · Accepted Answer · 2017-04-27 17:14:53Z

14

asyncio.get_event_loop returns the current loop. It pays no attention to the state of the loop. If you need a new loop after closing one, you can use asyncio.new_event_loop.

Be aware that getting a new loop won't affect subsequent calls to get_event_loop. If you'd like that to return your new loop instead of the original one (especially since you've probably closed it), you'll need to call asyncio.set_event_loop yourself.

import asyncio

async def f():
    await asyncio.sleep(0)

loop = asyncio.get_event_loop()
loop.run_until_complete(f())
loop.close()

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

edited Apr 27, 2017 at 17:14

answered Apr 27, 2017 at 3:33

dirn

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3 Comments

T. Bringazi Over a year ago

Thank you very much!!! That worked great. I had tried new_event_loop earlier, but I must have done something wrong during that experiment.

dirn Over a year ago

I neglected to mention set_event_loop. That may be related to the problems you ran into. I've updated the answer to include it.

turdus-merula Over a year ago

For posterity - most functions in package asyncio receive the optional parameter loop.

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