[Twisted-Python] Re: DeferredList and errbacks

David Bolen db3l at fitlinxx.com
Thu Apr 21 14:39:38 EDT 2005

Andrew Bennetts <andrew-twisted at puzzling.org> 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

-- David

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