https://docs.python.org/3/library/asyncio-dev.html#asyncio-coroutine-not-scheduled
18.5.9.6. Detect #coroutine objects never #scheduled
When a coroutine function is called and its result is not passed to ensure_future() or to the BaseEventLoop.create_task() method, the execution of the coroutine object will never be scheduled which is probably a bug. Enable the debug mode of asyncio to log a warning to detect it.
Example with the bug:
import asyncio
@asyncio.coroutine
def test():
print("never scheduled")
test()
Output in debug mode:
Coroutine test() at test.py:3 was never yielded from
Coroutine object created at (most recent call last):
File "test.py", line 7, in <module>
test()
The fix is to call the ensure_future() function or the BaseEventLoop.create_task() method with the coroutine object.