[Twisted-Python] question about deferred lists tutorial description

Josef Novak josef.robert.novak at gmail.com
Tue Mar 11 21:04:52 EDT 2008

 I have been using twisted intermittently for a couple of months, but
still don't feel like I have a good intuitive understanding of how to
get things done quickly, and efficiently with the framework.  Thus
I've been revisiting the tutorial documents and resolved to go through
them with a fine-toothed comb in an effort to really get my noggin
around things.
 In the deffered discussion:

 there is a section about defer.DeferredList, which contains code
examples and a subsequent note which -seems- to contradict the code
above it:

------------begin code from tutorial----------
The callback will be called with a list of the results of the
Deferreds it contains, like so:

def printResult(result):
    print result
deferred1 = defer.Deferred()
deferred2 = defer.Deferred()
deferred3 = defer.Deferred()
dl = defer.DeferredList([deferred1, deferred2, deferred3])
# At this point, dl will fire its callback, printing:
#     [(1, 'one'), (0, 'bang!'), (1, 'three')]
# (note that defer.SUCCESS == 1, and defer.FAILURE == 0)
------------end code from tutorial----------

Ok, so far, so good.  You can build a series of deferreds, stick them
in a DeferredList, add a callback to the list, *subsequently* add some
callbacks/errbacks to the individual deferreds in the DeferredList,
fire things up, and have the results of the *subsequently added*
callbacks/errbacks sent to the DeferredList callback and printed out.

However, the following Note regarding callbacks for DeferredList
objects seems to contradict the promised results of the above
codeblock, stating that:

"""If you want to apply callbacks to the individual Deferreds that go
into the DeferredList, you should be careful about when those
callbacks are added. The act of adding a Deferred to a DeferredList
inserts a callback into that Deferred (when that callback is run, it
checks to see if the DeferredList has been completed yet). The
important thing to remember is that it is this callback which records
the value that goes into the result list handed to the DeferredList's

Therefore, if you add a callback to the Deferred after adding the
Deferred to the DeferredList, the value returned by that callback will
not be given to the DeferredList's callback. To avoid confusion, we
recommend not adding callbacks to a Deferred once it has been used in
a DeferredList."""

Ok, so in fact, any callbacks/errbacks I need to add to deferreds
which will be added to a DeferredList object, need to be added Prior
to my adding the individual deferreds to the DeferredList object.  Or
so it would appear to my limited understanding.

I would greatly appreciate it if someone could resolve this apparent
contradiction for me, or if it is a mistake in the documentation,
please point out which one of the code block or the note is actually


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