這是因爲只有Task
銷燬而沒有檢索到結果時纔會引發異常。當您將Task
分配給全局變量時,它將始終具有活動引用,因此永遠不會被銷燬。有一個在ASYNCIO/futures.py文檔字符串是深入討論了這個:
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.
如果你想看到/處理異常,只是用add_done_callback
處理任務的結果,並採取一切必要的時候你會得到一個例外:
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()
謝謝,這實際上使我的項目更容易,指定一個特定任務的異常處理程序,而不是事件循環中的任何異常。 – 2014-12-04 17:24:35