[Twisted-Python] DeferredList.addDeferred behaviour

I have been playing with DeferredList if found that his behaviour was not what I expected,

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dfl = DeferredList([]) print dfl.called True dfl.addDeferred(d) d.called 0 print dfl.called True

The workaround it to first create the list of deferred instance and then instanciate with it the DeferredList. But what sense does have the addDeferred method then ? I this behaviour what is supposed to be ? rodrigob.