“Asyncio Event Loop is Closed” when getting loop

When trying to run the asyncio hello world code example given in the docs:

import asyncio

async def hello_world():
    print("Hello World!")

loop = asyncio.get_event_loop()
# Blocking call which returns when the hello_world() coroutine is done
loop.run_until_complete(hello_world())
loop.close()

I get the error:

RuntimeError: Event loop is closed

I am using python 3.5.3.

Answers:

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Method 1

On Windows seems to be a problem with EventLoopPolicy, use this snippet to work around it:

asyncio.set_event_loop_policy(asyncio.WindowsSelectorEventLoopPolicy())
asyncio.run(main())

Method 2

You have already called loop.close() before you ran that sample piece of code, on the global event loop:

>>> import asyncio
>>> asyncio.get_event_loop().close()
>>> asyncio.get_event_loop().is_closed()
True
>>> asyncio.get_event_loop().run_until_complete(asyncio.sleep(1))
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/.../lib/python3.6/asyncio/base_events.py", line 443, in run_until_complete
    self._check_closed()
  File "/.../lib/python3.6/asyncio/base_events.py", line 357, in _check_closed
    raise RuntimeError('Event loop is closed')
RuntimeError: Event loop is closed

You need to create a new loop:

loop = asyncio.new_event_loop()

You can set that as the new global loop with:

asyncio.set_event_loop(asyncio.new_event_loop())

and then just use asyncio.get_event_loop() again.

Alternatively, just restart your Python interpreter, the first time you try to get the global event loop you get a fresh new one, unclosed.

As of Python 3.7, the process of creating, managing, then closing the loop (as well as a few other resources) is handled for you when use asyncio.run(). It should be used instead of loop.run_until_complete(), and there is no need any more to first get or set the loop.

Method 3

…and just in case:

import platform
if platform.system()=='Windows':
    asyncio.set_event_loop_policy(asyncio.WindowsSelectorEventLoopPolicy())

If you ever deploy your code in the cloud it might avoid painful debug.

All methods was sourced from stackoverflow.com or stackexchange.com, is licensed under cc by-sa 2.5, cc by-sa 3.0 and cc by-sa 4.0