Something I just encountered is that with Python 3.7 + threads + unix, will fail with this error:

RuntimeError: Cannot add child handler, the child watcher does not have a loop attached

The solution is to make sure there's a asyncio event loop in the main thread and that the child watcher is activated:

# (at file level)
asyncio.get_current_loop()
asyncio.get_child_watcher()

(not necessary in 3.8+ as the default child watcher implementation is better)
Out of the scope this PR, but I feel like a mention in the docs should at least be considered. Or is it too niche a situation/use-case?

@Mattwmaster58 I'll add some tests for the Thread. Is that just running sync python on a thread use case?

Yeah, an example for this setup would gunicorn + flask with endpoints that use the sync interface.