The folks in the twisted community found the most wonderful trick.  Consider this typical example of event driven code.  Consider this example (proabably easier to read bottom to top).  The function enqueueTaskToDisplayURL depends on three callbacks to handle the usual eventualities (good, bad, wrapup).

from twisted.web import client
from twisted.internet import reactor
import sys

def shutdown(*whatever): reactor.stop()

def printPage(data): print data

def printError(failure):
    print >> sys.stderr, "Error:", failure.getErrorMessage()

def enqueueTaskToDisplayURL(url):
    get = client.getPage(url)
    get.addCallbacks(printPage,
                     errback=printError)
    get.addBoth(shutdown)

 if __name__ == "__main__":
    enqueueTaskToDisplayURL("http://example.com/")
    reactor.run()

The trick they found was to use generators of their promise abstraction (aka defer).  So we can write that example as so:

from twisted.internet import defer
from twisted.web import client
from twisted.internet import reactor
import sys

@defer.inlineCallbacks
def enqueueTaskToDisplayURL(url):
    try:
        data = yield client.getPage(url)
        print data
    except Exception, e:
        print >> sys.stderr, "Error:", e
        return;
    finally: reactor.stop()

if __name__ == "__main__":
    enqueueTaskToDisplayURL("http://example.com/")
    reactor.run()

The clever hack is done by @defer.inlineCallbacks, which works because this function (notice the yield) is a generator.  That just made be giggle.

I’ve only gotten into Python and Twisted in a part time way over the past few months.  I’m probably committing all kinds of sins here.