在事件循环中多次调用回调函数。

Question

我正在尝试理解下面代码的行为。我尝试注册一个回调函数trampoline，以便使用asyncio模块在事件循环中调用。trampoline got本身被多次调用，我确信这是为什么发生的。我的期望是，回调trampoline只会在0.5秒之后调用自己一次？

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：我也有一个异常，我也不确定：

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

Answer 1

问题是，当调用时，trampoline会将回调调度到自身。

这意味着trampoline将在0.5秒后被调用，在0.5秒后再重新安排对自身的回调

要只调用它一次，您应该从.call_later内部删除trampoline并将其移到外部：

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

loop.call_later(0.5, trampoline)
...

异常是由于使用.call_soon调用trampoline() (返回值：None)而不是trampoline (函数本身)。