On Fri, Dec 4, 2015 at 11:39 AM, Justin Mayfield <too...@gmail.com> wrote:
> Hi Guido, > > Yes, I'd be interested in that. > This would be a great way to get your hands dirty! > There are two things off the top of my head that I don't have a solid > grasp on. > > 1. The effect of catching BaseException unilaterally and its affect on > generator protocol. I'm not fluent enough in the Task mech to understand > if this would break things. > Hm, I don't think that BaseException is treated specially by the generator protocol. Now, there might be some BaseExceptions where catching them isn't going to help much. MemoryError comes to mind, but catching it is unlikely to make things worse -- they are already pretty bad at that point, and if it gets re-raised it'll likely eventually just bubble up the stack until the program exits -- just as if you don't catch it. The bottom line is that catching exceptions isn't going to mess with the interpreter's integrity, so whatever you do shouldn't overly worry you. > 2. Having seen the issue in asyncio and also aiohttp I assume there will > be a never ending list of libraries that sometimes catch BaseException and > some that do not. This may be unsolvable by trying to achieve consistency > everywhere unless we can somehow convince an entire community that there is > one, and only one, way to do exception handling in async code. That just > seems like an impossible task to me but your influence may tell another > story. > I can't force people to change their code. And I don't think we should worry about this -- if a library doesn't catch BaseException, it'll be dealt with by asyncio itself, and that should be sufficient. If for a particular use case it isn't, that is between the user and the library author. Dealing with BaseException properly in asyncio should give both users and library authors the confidence that this *can* be dealt with (as opposed to the current situation where asyncio itself doesn't really try, so why would other libraries bother). > I'll do some experiments this weekend and report back. Please advise if > you want an issue filed and I'll spend some time writing a test that fails. > It's always better to track a specific issue in a tracker than in a general mailing list. In this case I recommend using asyncio's own tracker ( https://github.com/python/asyncio) so we can refine the approach without bothering the other CPython developers. Thanks again for volunteering to look into this! --Guido > JM > > On Fri, Dec 4, 2015 at 12:08 PM Guido van Rossum <gu...@python.org> wrote: > >> Hi Justin, >> >> I think in the long run the event loop (and everything in asyncio) needs >> to be much more careful with BaseExceptions. Their treatment is uneven -- >> if we're writing "except Exception" they are not caught, but when we're >> writing "finally" or "except: ...; raise" they are caught. I think the >> right thing to do is to tread BaseException the same way we're treating >> Exception, *except* that in cases where we catch it and continue running >> the event loop we should probably still break out of the event loop right >> away. So a KeyboardError would still break out of the event loop (without >> executing the remaining scheduled callbacks; or perhaps after, similar to >> the way the new stop() behaves), but the event loop would be in a >> consistent state -- if you were to restart the loop the KeyboardError would >> be delivered to the callback to which an Exception class would be delivered. >> >> Hope this description makes sense. >> >> Maybe you're interested in improving asyncio by submitting a patch, given >> that you're running into this problem and so have a motivation as well as a >> use case that can drive the improvements? >> >> On Fri, Dec 4, 2015 at 12:54 AM, Justin Mayfield <too...@gmail.com> >> wrote: >> >>> I have an experimental library where I manage the lifecycle of event >>> loops for purposes of allowing async routines to operate inside a classical >>> generator (among other things). I create a new event loop for each use of >>> my routine and upon completion or GC this event loop is closed. >>> >>> My recent testing has left me a bit perplexed as to the right way to >>> deal with KeyboardInterrupt and possibly other BaseException exceptions >>> that halt execution of an event loop. I've read a few posts on this list >>> suggesting that the basic rule of thumb employed within asyncio itself, is >>> to only catch Exception because BaseException is generally considered >>> unrecoverable (or coro protocol as in GeneratorExit). However I seem to >>> have found several instances where this rule is not followed and in the >>> except clause, further calls are made that expect the event loop to be in >>> good standing. >>> >>> Because I close the event loop when my routine is done/interrupted the >>> eventual GeneratorExit exception thrown by the garbage collector triggers >>> some cleanup/close routines in asyncio itself (as well as aiohttp) that >>> eventually find themselves staring at RuntimeError('Event loop is closed'). >>> >>> My question to the community is if I'm witnessing bugs in these places >>> where naked except clauses are used to perform cleanup/closing actions on >>> an event loop or if I'm fundamentally doing something wrong. I hope it's >>> not the latter as I've been pretty happy with the results of my experiment >>> baring these corner cases. >>> >>> [Code in question is here: >>> https://github.com/mayfield/cellulario/blob/master/cellulario/iocell.py] >>> >>> Example traceback.. >>> >>> Traceback (most recent call last): >>> >>> File >>> "/usr/local/Cellar/python3/3.5.0/Frameworks/Python.framework/Versions/3.5/lib/python3.5/asyncio/base_events.py", >>> line 692, in _create_connection_transport >>> >>> yield from waiter >>> >>> GeneratorExit >>> >>> >>> During handling of the above exception, another exception occurred: >>> >>> >>> Traceback (most recent call last): >>> >>> File "/Users/mayfield/project/syndicate/syndicate/adapters/async.py", >>> line 74, in request >>> >>> result = yield from asyncio.wait_for(r, timeout, loop=self.loop) >>> >>> File >>> "/usr/local/Cellar/python3/3.5.0/Frameworks/Python.framework/Versions/3.5/lib/python3.5/asyncio/tasks.py", >>> line 359, in wait_for >>> >>> return (yield from fut) >>> >>> File >>> "/usr/local/lib/python3.5/site-packages/aiohttp-0.18.3-py3.5-macosx-10.10-x86_64.egg/aiohttp/client.py", >>> line 456, in __iter__ >>> >>> resp = yield from self._coro >>> >>> File >>> "/usr/local/lib/python3.5/site-packages/aiohttp-0.18.3-py3.5-macosx-10.10-x86_64.egg/aiohttp/client.py", >>> line 173, in _request >>> >>> conn = yield from self._connector.connect(req) >>> >>> File >>> "/usr/local/lib/python3.5/site-packages/aiohttp-0.18.3-py3.5-macosx-10.10-x86_64.egg/aiohttp/connector.py", >>> line 289, in connect >>> >>> transport, proto = yield from self._create_connection(req) >>> >>> File >>> "/usr/local/lib/python3.5/site-packages/aiohttp-0.18.3-py3.5-macosx-10.10-x86_64.egg/aiohttp/connector.py", >>> line 557, in _create_connection >>> >>> server_hostname=hinfo['hostname'] if sslcontext else None) >>> >>> File >>> "/usr/local/Cellar/python3/3.5.0/Frameworks/Python.framework/Versions/3.5/lib/python3.5/asyncio/base_events.py", >>> line 669, in create_connection >>> >>> sock, protocol_factory, ssl, server_hostname) >>> >>> File >>> "/usr/local/Cellar/python3/3.5.0/Frameworks/Python.framework/Versions/3.5/lib/python3.5/asyncio/base_events.py", >>> line 694, in _create_connection_transport >>> >>> transport.close() >>> >>> File >>> "/usr/local/Cellar/python3/3.5.0/Frameworks/Python.framework/Versions/3.5/lib/python3.5/asyncio/selector_events.py", >>> line 566, in close >>> >>> self._loop.call_soon(self._call_connection_lost, None) >>> >>> File >>> "/usr/local/Cellar/python3/3.5.0/Frameworks/Python.framework/Versions/3.5/lib/python3.5/asyncio/base_events.py", >>> line 453, in call_soon >>> >>> handle = self._call_soon(callback, args) >>> >>> File >>> "/usr/local/Cellar/python3/3.5.0/Frameworks/Python.framework/Versions/3.5/lib/python3.5/asyncio/base_events.py", >>> line 462, in _call_soon >>> >>> self._check_closed() >>> >>> File >>> "/usr/local/Cellar/python3/3.5.0/Frameworks/Python.framework/Versions/3.5/lib/python3.5/asyncio/base_events.py", >>> line 289, in _check_closed >>> >>> raise RuntimeError('Event loop is closed') >>> >>> >> >> >> -- >> --Guido van Rossum (python.org/~guido) >> > -- --Guido van Rossum (python.org/~guido)
