import asyncio
import traceback
from threading import Thread


class Test(Thread):
    def __init__(self):
        super().__init__()
        self.loop = asyncio.new_event_loop()

    async def getaddrinfo(self, loop):
        try:
            print(await loop.getaddrinfo("www.google.com", 8333))
        except Exception:
            print(traceback.format_exc())

    def run(self):
        loop = self.loop
        asyncio.set_event_loop(loop)
        asyncio.run_coroutine_threadsafe(self.getaddrinfo(loop), loop)
        loop.run_forever()


test = Test()
test.start()

Executing the previous code throws RuntimeError("can't register atexit after shutdown") only in python3.9. Strangely it doesn't happen when I execute the code in interactive mode. 

I think this is related to https://bugs.python.org/issue41962

Your snippet exits just after starting.
`getaddrinfo()` is called at the interpreter shutdown state.

I'm not sure that do you expect to get here.  We can put some effort and raise an error with another message but cannot make the example working.

I suggest just closing the issue; nothing to do here.

Why do you say that `getaddrinfo()` is called at the interpreter shutdown state? On my machine it works and the output is [(<AddressFamily.AF_INET: 2>, <SocketKind.SOCK_STREAM: 1>, 6, '', ('216.58.208.132', 8333)), (<AddressFamily.AF_INET: 2>, <SocketKind.SOCK_DGRAM: 2>, 17, '', ('216.58.208.132', 8333)), (<AddressFamily.AF_INET: 2>, <SocketKind.SOCK_RAW: 3>, 0, '', ('216.58.208.132', 8333)), (<AddressFamily.AF_INET6: 10>, <SocketKind.SOCK_STREAM: 1>, 6, '', ('2a00:1450:4002:805::2004', 8333, 0, 0)), (<AddressFamily.AF_INET6: 10>, <SocketKind.SOCK_DGRAM: 2>, 17, '', ('2a00:1450:4002:805::2004', 8333, 0, 0)), (<AddressFamily.AF_INET6: 10>, <SocketKind.SOCK_RAW: 3>, 0, '', ('2a00:1450:4002:805::2004', 8333, 0, 0))], which is printed on the console

Because you have `test.start()` and immediately finish the script.

The interpreted goes to *shutdown* state and implicitly waits for `test.join()` because the test thread is not a daemon.

Ok but then why does it work in python3.8 and not in python 3.9?

Because it did work in a gray area.
The finalization logic has slightly changed, some tricks that were working before now fails.
Sure, other undocumented tricks start working :)

The rule of thumb: please stop and close the asyncio loop before exiting the program. This rule allows for avoiding situations like this.