On Tue, Apr 05, 2005 at 04:13:36PM -0700, Lenny G Arbage wrote:
I just noticed [what appears to me to be] a contradiction in the twisted documentation. The "Using Deferreds" howto [1] says, "A standard DeferredList will never call errback," while the "Twisted FAQ" [2] states, "Always add errbacks!" in response to the question, "My Deferred or DeferredList never fires, so my program just mysteriously hangs! What's wrong?"
So, which is it?
I suppose what this is getting at is that each deferred in a DeferredList should have an errback attached separately, but this isn't super-clear to me. Is this the right way to interpret the above statements?
By default, DeferredList waits for all its Deferreds to fire (with a result or an error), and returns a big list of results of the form: [(success_bool, value), (success_bool, value), ...] Where 'success_bool' is True if the corresponding Deferred succeeded (i.e. fired a callback), and False if it failed (i.e. fired an errback). 'value' will be a result or a Failure instance accordingly. So a standard DeferredList will never call errback, because it includes errors in the result passed to its callback. Error handling in this case could be handled by the user of the DeferredList, because it would have access to any underlying failures via the callback. But you're correct, the original deferreds would still need errbacks attached -- or pass consumeErrors=True to DeferredList, and it will do this for you. And of course, the situation is a bit different again if you pass fireOnOneErrback=True... You're right that the docs are a bit confusing here. Please file a bug about it at http://twistedmatrix.com/bugs. The "Other Behaviours" section of http://twistedmatrix.com/documents/current/howto/defer tries to address this, but perhaps we could do better. -Andrew.