Mine

# showing who stopped the reactor

def doSomethingAndStop(name):
  reactor.callLater(0, doDeferredThing, name)

def doDeferredThing(name):
  d = What()
  d.addBoth(stopNow, name)

def stopNow(result):
  reactor.stoppedBy = result
  reactor.stop()

doSomethingAndStop("fred")
reactor.run()
print reactor.stoppedBy
doSomethingAndStop("wilma")
reactor.run()
print reactor.stoppedBy