Re: [Twisted-Python] Re: DeferredList and errbacks

On 21 Apr 2005 14:39:38 -0400, David Bolen wrote:

Andrew Bennetts writes:

...

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.

Note however, that while the DeferredList will never errback, it doesn't stop the errback chains of the individual deferreds. It inserts itself into those chains to gather the information, but continues to let the failure propagate.

So if you expect the DeferredList to be the final aggregation of information for all of the individual deferreds, but don't want unterminated errbacks (which often, but not always, result in default logging), you still need to install additional errbacks on each of the deferreds you hand to the DeferredList. And you need to add them after the deferreds are installed in the DeferredList or its the result of your individual errbacks that will flow to the DeferredList result.

Alternatively, pass consumeErrors=True to DeferredList() and it will do something equivalent for all Deferreds you put into it. Jp