[Twisted-Python] The Real Error (tm) [was Re: how to write a safe catch-all]

Phil Mayers p.mayers at imperial.ac.uk
Fri Oct 1 09:28:26 EDT 2010

On 01/10/10 14:19, Chris Withers wrote:
> On 30/09/2010 18:01, Phil Mayers wrote:
>> It is more than a little confusing, and I'm sure frustrating.
>> I've tried to produce something like this locally, but cannot.
> The very first message in this thread (28th Sept, 14:48) and my email of
> 30th Sept, 14:36 both had scripts attached which do exactly this..

If your examples scripts genuinely reflect the problem (a bit of code, 
outside your control, is raising an exception in a context where the 
error doesn't flow back to user code) then Glyph is (of course!) right - 
you're not going to be able to capture and log the error "nicely". It 
will be propagated upwards until it hits the reactor.

>> You want this loop function to catch&   log all exceptions resulting from
>> work it initiates.
> Yep, errbacks, internal twisted bugs, whatever. As long as they're
> logged by something, preferabyl the try-except above, I don't mind.
> What absolutely must not cannot ever happen is for the scheduler to die ;-)

Now *this* you should be able to do.

A LoopingCall will only die (i.e. stop permanently) if you:

  1. Raise an exception
  2. Return a deferred which errback's

(The former gets turned into the latter)

If you ensure your loop never ever does this, it'll run forever.

HOWEVER: as JP has mentioned, a LoopingCall *will* pause if you return a 
deferred, until that deferred callback's (or errback's, then it dies)

So this:

def loop():

...should never ever die (because it can't raise an exception) or stop 
(because it doesn't return a deferred)

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