[Twisted-Python] Tracebacks being dropped from exceptions when using inlineCallbacks
glyph at divmod.com
glyph at divmod.com
Sat Jan 17 02:54:45 EST 2009
On 02:31 am, radix at twistedmatrix.com wrote:
>On Fri, Jan 16, 2009 at 7:28 PM, Phil Christensen
><phil at bubblehouse.org> wrote:
>Keeping references to tracebacks still has many potential pitfalls.
>It's a fundamental problem: tracebacks refer to all their frames,
>which refer to all their locals; this makes it *really* easy to create
>uncollectable cycles if you have any __del__ methods *anywhere*. It's
>just not safe to keep the traceback reference in the general case.
>Failure needs to work with arbitrary code.
This used to be pretty much 100% of the time, since Deferred itself used
to have a __del__, so if you had a Deferred anywhere on the stack you
were dead. Nowadays it's a little better than that, but a surprising
number of objects from elsewhere are uncollectable.
>>Originally all __getstate__ did was stringify the object's state,
>>probably why cleanFailure calls it directly. It looks to me like most
>>code from __getstate__ should really be moved into a separate method,
>>would be called from cleanFailure as well as __getstate__.
Nevertheless cleanFailure will still need to remove the traceback, for
the reasons that Chris mentioned.
>Sure, that would be a slightly better factoring.
Would that help, though?
I don't fully understand the issue, so this is probably wrong in some
obvious way, but: do we really need to hang on to a real traceback
object to resolve the issue in question? It sounds like the real issue
here is that we don't have a channel to communicate a "cleaned
traceback" (i.e., failure with stringified frames, but no traceback)
from one bit of code to another?
>>>Note Beginning with Python 2.2, such cycles are automatically
>>>when garbage collection is enabled and they become unreachable, but
>>>remains more efficient to avoid creating cycles.
I'm not sure what Python documentation you're quoting here, but that's
not accurate in the general case. For example, see
"Objects that have __del__() methods and are part of a reference
cycle cause the entire reference cycle to be uncollectable".
>>I'd appreciate any feedback.
Two bits of feedback:
1. More of this discussion should be on a ticket. The mailing list is
good for getting discussions started and figuring out what needs to be
done, but now we're talking about the technical specifics of resolving a
specific problem. Someone (terry?) should take the specifics in the
email that started this thread and file an appropriate ticket.
2. When filing that ticket, I'd really like a full working example of
inlineCallbacks not showing a traceback to play with, not just examples
of portions of the problem. The obvious example I tried,
from twisted.internet.defer import inlineCallbacks
actually prints a nice descriptive traceback.
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