Asyncio: Weirdness of Task exception was never retrieved

问题

Lets assume I have a simple code:

import asyncio


async def exc():
    print(1 / 0)


loop = asyncio.get_event_loop()

loop.create_task(exc())

try:
    loop.run_forever()
except KeyboardInterrupt:
    loop.stop()
    loop.close()

If I run it, I get error message immediately

Task exception was never retrieved
future: <Task finished coro=<exc() done, defined at qq.py:4> exception=ZeroDivisionError('division by zero',)>
Traceback (most recent call last):
  File "qq.py", line 5, in exc
    print(1 / 0)
ZeroDivisionError: division by zero

But, if I change loop.create_task(exc()) to task = loop.create_task(exc())

I'll get the same error message after click ctrl+c

Why does task assignment change the time of output of error?

回答1:

A Exception in the Task (of underlying asyncio.Future to be precise) can be retrieved with Future.exception(). If it's not retrieved, the exception will be handled at release of the Future object with eventloop's call_exception_handler.

So, as @dim pointed, while the Task has reference (assigned to variable in your case) it's not going be freed, therefore del task_future won't be called, loop's handler won't be executed either.

来源：https://stackoverflow.com/questions/46890646/asyncio-weirdness-of-task-exception-was-never-retrieved