[issue33403] asyncio.tasks.wait does not allow to set custom exception when return_when=FIRST_EXCEPTION

[pyneda]

New submission from pyneda <rogerpineda13 at gmail.com>:

A possible use case (that at least I couldn't find how to solve) is the possibility to cancel a bunch of futures/coroutine objects which are being awaited using asyncio.tasks.wait() by one of the running coroutines when a task succeded or a specific condition happens by just raising an specific exception.

At the moment this can be done but it will cancel all the coroutines with any exception that is raised and at some occasions this may not be desired.

A simple example:

async def example(num):
    if x == 5:
        raise Exception('Exception that does not cancel')
    elif x == 15:
        raise CancelException()

tasks = [asyncio.ensure_future(example(x)) for x in range(20)]
done, pending = await asyncio.wait(tasks, return_when=FIRST_EXCEPTION, return_on=CancelException)

This wouldn't cancel everything when a normal exception is raised, instead it will when the exception raised is the one that the user expects to be raised in order to cancel everything that is pending. 

In addition, if the user does not specify the Exception type, it uses default Exception so it would keep working exactly as now.

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue33403>
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