On 04/20/2011 05:28 AM, David wrote:
> Hi,
>
> I have a hard time figuring out error handling with deferred in twisted.
> More exactly, I don't understand how to always get meaningful tracebacks
> to understand where the error actually happened. For a simple example:
>
> import sys
>
> import twisted.web.client
>
> from twisted.internet import defer
> from twisted.internet import reactor
> from twisted.python import log
>
> def remote_call():
> # No process bound to 8083 -> connection refused
> d = twisted.web.client.getPage("http://localhost:8083";)
> return d
>
> def main():
> d = remote_call()
> def _stop(arg):
> reactor.stop()
> d.addBoth(_stop)
>
> log.startLogging(sys.stdout)
> reactor.callWhenRunning(main)
> reactor.run()
>
> This will simply print no error in the log:
>
> 2011-04-20 12:37:40+0900 [-] Log opened.
> 2011-04-20 12:37:40+0900 [-] Starting factory<HTTPClientFactory:
> http://localhost:8083>
> 2011-04-20 12:37:40+0900 [HTTPPageGetter,client] Stopping factory
> <HTTPClientFactory: http://localhost:8083>
> 2011-04-20 12:37:40+0900 [-] Main loop terminated.
>
> This already bothers me at a fundamental level, because it means it is
> very easy to "swallow" errors without being aware of it. Is there a
> "systematic" solution to this issue, or am I condemned to handle errors
> systematically everywhere in my code ?
>
> Now, if I add an errback for logging purpose:
>
> def main():
> d = remote_call()
> def _stop(arg):
> reactor.stop()
> d.addErrback(log.err)
> d.addBoth(_stop)
>
> I get something like:
>
> 2011-04-20 12:38:35+0900 [-] Log opened.
> 2011-04-20 12:38:35+0900 [-] Starting factory<HTTPClientFactory:
> http://localhost:8083>
> 2011-04-20 12:38:35+0900 [HTTPPageGetter,client] Unhandled Error
> Traceback (most recent call last):
> Failure: twisted.web.error.Error: 404 Not Found
>
> 2011-04-20 12:38:35+0900 [HTTPPageGetter,client] Stopping factory
> <HTTPClientFactory: http://localhost:8083>
> 2011-04-20 12:38:35+0900 [-] Main loop terminated.
>
> I do get an error, but I don't get a traceback. Interestingly enough, if
> I use printTraceback:
>
> def main():
> d = remote_call()
> def _stop(arg):
> reactor.stop()
> def log_error(failure):
> log.err(failure.printTraceback())
> return failure
> d.addErrback(log_error)
> d.addBoth(_stop)
>
> I don't get any traceback either.
As other people have pointed out, you're discarding the failure.
This:
def on_error(failure):
return
d = some_call()
d.addErrback(on_error)
...is equivalent to synchronous code of the form:
try:
some_call()
except:
pass
...i.e. silently eating all exceptions. The latter is usually bad
python, and so the former is usually bad Twisted ;o)
_______________________________________________
Twisted-Python mailing list
Twisted-Python@twistedmatrix.com
http://twistedmatrix.com/cgi-bin/mailman/listinfo/twisted-python
