If a signal handler callback is registered on an event loop, and the event loop has close() called on it, the close will fail.

Exception:
Exception in thread Thread-1:
Traceback (most recent call last):
  File "/home/nic/.pyenv/versions/3.7.0/lib/python3.7/threading.py", line 917, in _bootstrap_inner
    self.run()
  File "/home/nic/.pyenv/versions/3.7.0/lib/python3.7/threading.py", line 865, in run
    self._target(*self._args, **self._kwargs)
  File "/home/nic/tmp/signal_event_loop_bug.py", line 9, in do_loop
    loop.close()
  File "/home/nic/.pyenv/versions/3.7.0/lib/python3.7/asyncio/unix_events.py", line 58, in close
    self.remove_signal_handler(sig)
  File "/home/nic/.pyenv/versions/3.7.0/lib/python3.7/asyncio/unix_events.py", line 147, in remove_signal_handler
    signal.signal(sig, handler)
  File "/home/nic/.pyenv/versions/3.7.0/lib/python3.7/signal.py", line 47, in signal
    handler = _signal.signal(_enum_to_int(signalnum), _enum_to_int(handler))
ValueError: signal only works in main thread

Code:
import asyncio
import signal
import threading

def mysighandler():
    pass

def do_loop(loop):
    loop.close()

loop = asyncio.new_event_loop()
loop.add_signal_handler(signal.SIGINT, mysighandler)
t = threading.Thread(target=do_loop, args=(loop,))
t.start()
t.join()