"Your script contains a bug (there is no definition of 'exec_fn')"

Told you I wasn't much of a Python developer ;-) 
This was just copy-pasted from spawn.py and I missed the fact that exec_fn() is not a library function.

Your last comment wasn't clear - do you see this problem on macOS? Is it only after you fixed the example? For me it hangs with the bug, as the problem occurs before the call to exec_fn().