A callback function is called multiple times in event loop

Question

I am trying to understand the behavior of the code below. I tried to register a callback function trampoline to be called in an event loop using asyncio module. The trampoline got itself is called multiple times, which I am sure why that happened please. My expectation was the callback trampoline will call itself only once later after 0.5 seconds?

import asyncio import datetime loop = asyncio.get_event_loop() def print_date(): print(datetime.datetime.now()) def trampoline(name: str = "") -> None: print("name:", end=" ") print_date() loop.call_later(0.5, trampoline, name) loop.call_soon(trampoline()) loop.call_later(8, loop.stop) loop.run_forever()

Output: I also have an exception, which also I am not sure about:

name: 2022-11-11 16:29:04.335313 Exception in callback None() handle: <Handle> Traceback (most recent call last): File "C:\Anaconda3\lib\asyncio\events.py", line 81, in _run self._context.run(self._callback, *self._args) TypeError: 'NoneType' object is not callable name: 2022-11-11 16:29:04.850481 name: 2022-11-11 16:29:05.363303 name: 2022-11-11 16:29:05.872649 name: 2022-11-11 16:29:06.383004 name: 2022-11-11 16:29:06.898766 name: 2022-11-11 16:29:07.413907 name: 2022-11-11 16:29:07.915795 name: 2022-11-11 16:29:08.428609 name: 2022-11-11 16:29:08.939604 name: 2022-11-11 16:29:09.454111 name: 2022-11-11 16:29:09.966438 name: 2022-11-11 16:29:10.470832 name: 2022-11-11 16:29:10.981675 name: 2022-11-11 16:29:11.496746 name: 2022-11-11 16:29:12.011841 Process finished with exit code 0

"My expectation was the callback trampoline will call itself only once later after 0.5 seconds". Yes, and what happens when that second call to trampoline takes place? — Carcigenicate
– Carcigenicate, Commented Nov 11, 2022 at 21:40
I guess you don‘t want to call trampoline() but instead pass the function? — tfeldmann
– tfeldmann, Commented Nov 11, 2022 at 21:42
@tfeldmann so you're saying it should just be loop.call_soon(trampoline) right? — Random Davis
– Random Davis, Commented Nov 11, 2022 at 21:55

Pignotto · Accepted Answer · 2022-11-11 23:41:11Z

The issue is that trampoline schedules a callback to itself when called.

That means that trampoline will be called after 0.5 seconds, rescheduling again a callback to itself for 0.5 seconds later

To have it only called once, you should remove .call_later from inside trampoline and move it outside:

... def trampoline(name: str = "") -> None: print("name:", end=" ") print_date() loop.call_later(0.5, trampoline) ...

The exception is due to calling .call_soon with trampoline() (the return value: None) instead of trampoline (the function itself)

Thank you. Why it does stop though after some calls? In the code, loop.run_forever() was used, so should not trampoline() run forever?
I guess it's because we call loop.call_later(8, loop.stop) that will stop the event loop.

Collectives™ on Stack Overflow

A callback function is called multiple times in event loop

1 Answer 1

2 Comments

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

2 Comments

Related