创建后异步任务被存储,任务的异常被静音

问题描述 投票:7回答:2

我在项目中使用asyncio,并遇到了这种奇怪的行为。

import asyncio

def schedule_something():
    global f
    tsk = asyncio.async(do_something())
    f = tsk #If this line is commented out, exceptions can be heard.

@asyncio.coroutine
def do_something():
    raise Exception()

loop = asyncio.get_event_loop()
loop.call_soon(schedule_something)
loop.run_forever()
loop.close()

由于某些原因,在调用asyncio.async()时存储结果任务会阻止异常执行任何操作。

有人可以阐明这种情况吗?我需要有一种方法可以在当前项目中捕获异常。

python python-asyncio
2个回答
7
投票

这是因为仅在Task被销毁而没有检索到其结果的情况下才会引发异常。当将Task分配给全局变量时,它将始终具有有效的引用,因此不会被破坏。在asyncio / futures.py中有一个文档字符串,对此进行了详细说明:

class _TracebackLogger:
    """Helper to log a traceback upon destruction if not cleared.

    This solves a nasty problem with Futures and Tasks that have an
    exception set: if nobody asks for the exception, the exception is
    never logged.  This violates the Zen of Python: 'Errors should
    never pass silently.  Unless explicitly silenced.'

    However, we don't want to log the exception as soon as
    set_exception() is called: if the calling code is written
    properly, it will get the exception and handle it properly.  But
    we *do* want to log it if result() or exception() was never called
    -- otherwise developers waste a lot of time wondering why their
    buggy code fails silently.

    An earlier attempt added a __del__() method to the Future class
    itself, but this backfired because the presence of __del__()
    prevents garbage collection from breaking cycles.  A way out of
    this catch-22 is to avoid having a __del__() method on the Future
    class itself, but instead to have a reference to a helper object
    with a __del__() method that logs the traceback, where we ensure
    that the helper object doesn't participate in cycles, and only the
    Future has a reference to it.

    The helper object is added when set_exception() is called.  When
    the Future is collected, and the helper is present, the helper
    object is also collected, and its __del__() method will log the
    traceback.  When the Future's result() or exception() method is
    called (and a helper object is present), it removes the the helper
    object, after calling its clear() method to prevent it from
    logging.

如果您想查看/处理异常,只需使用add_done_callback处理任务的结果,并在获得异常时执行所有必要的操作:

add_done_callback

0
投票

谢谢,@ dano。这是import asyncio def handle_result(fut): if fut.exception(): fut.result() # This will raise the exception. def schedule_something(): global f tsk = asyncio.async(do_something()) tsk.add_done_callback(handle_result) f = tsk @asyncio.coroutine def do_something(): raise Exception() loop = asyncio.get_event_loop() loop.call_soon(schedule_something) loop.run_forever() loop.close() 的替代品,它可以自动执行此操作-

asyncio.create_task

给定示例的更新版本-

def create_task(coro):
    def on_task_done(fut):
        if fut.exception():
            fut.result()

    async def task_wrapper():
        task.remove_done_callback(on_task_done)
        return await task

    task = asyncio.create_task(coro)
    task.add_done_callback(on_task_done)

    return task_wrapper()
async def do_something():
    raise Exception()


async def schedule_something():
    global f
    tsk = create_task(do_something())
    f = tsk  # If this line is commented out, exceptions can be heard.


asyncio.run(schedule_something())
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