My Deferred or DeferredList never fires, so my program just mysteriously hangs! Whats wrong?
It really depends on what your program is doing, but the most common cause is this: it is firing — but it’s an error, not a success, and you have forgotten to add an errback, so nothing happens. Always add errbacks! The reason Deferred can’t automatically show your errors is because a Deferred can still have callbacks and errbacks added to it even after a result is available — so we have no reasonable place to put a logging call that wouldn’t result in spurious tracebacks that are handled later on. There is a facility for printing tracebacks when the Deferreds are garbage collected — call defer.setDebugging(True) to enable it.
