#!/usr/bin/python2.3 from twisted.internet import defer from twisted.python import failure, util """ this example shows an important concept that many deferred newbies (myself included) have trouble understanding. when an error occurs in a callback, the first errback after the error occurs will be the next method called. (in the next example we'll see what happens in the 'chain' after an errback) """ class Counter(object): num = 0 def handleFailure(f): print "errback" print "we got an exception: %s" % (f.getTraceback(),) f.trap(RuntimeError) def handleResult(result): Counter.num += 1 print "callback %s" % (Counter.num,) print "\tgot result: %s" % (result,) return "yay! handleResult was successful!" def failAtHandlingResult(result): Counter.num += 1 print "callback %s" % (Counter.num,) print "\tgot result: %s" % (result,) print "\tabout to raise exception" raise RuntimeError, "whoops! we encountered an error" def behindTheScenes(result): # equivalent to d.callback(result) # now, let's make the error happen in the first callback if not isinstance(result, failure.Failure): # ---- callback try: result = failAtHandlingResult(result) except: result = failure.Failure() else: # ---- errback pass # note: this callback will be skipped because # result is a failure if not isinstance(result, failure.Failure): # ---- callback try: result = handleResult(result) except: result = failure.Failure() else: # ---- errback pass if not isinstance(result, failure.Failure): # ---- callback pass else: # ---- errback try: result = handleFailure(result) except: result = failure.Failure() def deferredExample(): d = defer.Deferred() d.addCallback(failAtHandlingResult) d.addCallback(handleResult) d.addErrback(handleFailure) d.callback("success") if __name__ == '__main__': behindTheScenes("success") print "\n-------------------------------------------------\n" Counter.num = 0 deferredExample()