[Twisted-Python] How to show traceback of "Unhandled error"?

[Shiyao MA]

Thanks,

That logger really helps.

Best,

On Sat, May 19, 2018 at 10:53 AM, Mark Williams <mrw at enotuniq.org> wrote:
> This is an unfortunate implementation detail that you can address by starting the logging system.
>
> Try this:
>
>
> from twisted.logger import globalLogBeginner, textFileLogObserver
> from twisted.internet.defer import Deferred
> import sys
>
> globalLogBeginner.beginLoggingTo([textFileLogObserver(sys.stdout)])
>
> def raiseErr(what):
>     raise Exception(what)
>
> d = Deferred()
> d.addCallback(raiseErr)
> d.callback("asdf")
>
>
> You should see a traceback.
>
> For future reference, the important lines are these:
>
>
> from twisted.logger import globalLogBeginner, textFileLogObserver
> import sys
> globalLogBeginner.beginLoggingTo([textFileLogObserver(sys.stdout)])
>
>
> They should be executed before any code that uses Deferreds.  twistd and the newer twist Twisted application runners automatically run these for you before starting your Service.  This is a great reason to write Twisted application plugins:
>
> https://twistedmatrix.com/documents/current/core/howto/tap.html
>
> Of course it would be better to not have to do anything at all to see important information related to errors; the default behavior of Deferred makes things harder to understand than they have to be.  Fortunately, good progress has been made on this issue:
>
> https://twistedmatrix.com/trac/ticket/9333
>
> The associated PR needs a careful review to ensure its handling of unicode is correct.  Any takers?
>
> On Fri, May 18, 2018, at 1:56 AM, Shiyao MA wrote:
>> Hi,
>>
>> Running the following script prints "Unhandled Error", but doesn't
>> contain the traceback of where the exception is raised.  In a massive
>> codebase, it's hard to locate where is the malfunctioning code is.
>>
>>
>> from twisted.internet.defer import Deferred
>>
>> def raiseErr(what):
>>     raise Exception(what)
>>
>> d = Deferred()
>> d.addCallback(raiseErr)
>> d.callback("asdf")
>>
>> # how to show the traceback without manually adding an errback at the end?
>> # adding at the *end* of a deferred is hard as we don't know when the
>> client stops adding callbacks.
>>
>>
>>
>>
>> --
>> Best,
>> Shiyao
>>
>> _______________________________________________
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>> Twisted-Python at twistedmatrix.com
>> https://twistedmatrix.com/cgi-bin/mailman/listinfo/twisted-python
>
>
> --
>   Mark Williams
>   mrw at enotuniq.org
>
> _______________________________________________
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-- 
Best,
Shiyao