Deferreds - Example

>>> def printFailure(failure): failure.printTraceback()
...
>>> d = Deferred()
>>> d.addCallback(square).addCallback(reciprocal)
<Deferred at 0x40216d2c>
>>> d.addCallback(printResult)
<Deferred at 0x40216d2c>
>>> d.addErrback(printFailure) 
<Deferred at 0x40216d2c>
>>> d.callback(0)
Traceback (most recent call last):
[...]
exceptions.ZeroDivisionError: float division