SynchronousOnlyOperation is raised for non-running event loops on Python 3.7+.

[wbastian-bh]

Env
Django 3.2.9+
Python 3.7+

Overview
With this commit (​https://github.com/django/django/commit/53fad80ffe16ab4edb713b1ef0090d0fcf63565a), which was included with the 3.2.9 release if we're on PY3.7+, we raise SynchronousOnlyOperation when asyncio.get_running_loop returns an object without checking event_loop.is_running().

It appears that asyncio.get_running_loop can return non-running loops, as observed by including a logging statement before raising the SynchronousOnlyOperation.

If my understanding is correct, get_running_loop should only be returning running loops, and is not.

Curious if we can continue to leverage event_loop.is_running() in all cases.

Observation Example

import asyncio
import functools
import os
​
from django.core.exceptions import SynchronousOnlyOperation
from django.utils.version import PY37
​
​
if PY37:
    get_running_loop = asyncio.get_running_loop
else:
    get_running_loop = asyncio.get_event_loop
​
​
def async_unsafe(message):
    """
    Decorator to mark functions as async-unsafe. Someone trying to access
    the function while in an async context will get an error message.
    """
    def decorator(func):
        @functools.wraps(func)
        def inner(*args, **kwargs):
            if not os.environ.get('DJANGO_ALLOW_ASYNC_UNSAFE'):
                # Detect a running event loop in this thread.
                try:
                    event_loop = get_running_loop()
                except RuntimeError:
                    pass
                else:
                    if PY37 or event_loop.is_running():
                        print(f"raising SynchronousOnlyOperation on {event_loop} where is_running = {event_loop.is_running()}")
                        raise SynchronousOnlyOperation(message)
            # Pass onwards.
            return func(*args, **kwargs)
        return inner
    # If the message is actually a function, then be a no-arguments decorator.
    if callable(message):
        func = message
        message = 'You cannot call this from an async context - use a thread or sync_to_async.'
        return decorator(func)
    else:
        return decorator

Observation Output

raising SynchronousOnlyOperation on <_UnixSelectorEventLoop running=False closed=False debug=False> where is_running = False

Steps to reproduce

1. Have a non-running event loop present and do just about anything in Django
2. See SynchronousOnlyOperation raised

[Mariusz Felisiak]

Thanks for the report, however according to ​Python docs asyncio.get_running_loop():

"Return the running event loop in the current OS thread.
Raise a RuntimeError if there is no running event loop."

If asyncio.get_running_loop() returns a non-running event loop for you, I'd consider this an issue in Python, not in Django itself.