twgs-proxy/examples/defer_alreadycallederror.py

#!/usr/bin/env python


# https://twistedmatrix.com/documents/16.2.0/core/howto/defer.html

from twisted.internet import reactor, defer
from pprint import pprint

def getData(inputData):
    print('getData({0})'.format(inputData))
    deferred = defer.Deferred()
    # simulate a delayed result by asking reactor
    # to fire in 2 seconds with inputData * 3 as result
    print("return result (x*3) in 2 sec...")
    print(deferred)
    reactor.callLater(2, deferred.callback, inputData * 3)
    print(deferred)
    reactor.callLater(3, deferred.callback, inputData * 3)
    print(deferred)
    return deferred

def cbPrint(result):
    print("cbPrint({0})".format(result))
    # doing eferred, or the return result + 1 works
    print("return result (x+1) in 1 sec...")
    deferred = defer.Deferred()
    reactor.callLater(1, deferred.callback, result + 1)
    return deferred
    # return result + 1

def cbPrint2(result):
    print("cbPrint2({0})".format(result))
    print("return result (x+2) ...")
    # print("(this isn't a defer)")    # it is now!  :P
    # return a Deferred object already called back with the value of result
    return defer.succeed(result + 2)
    # return result + 2

d = getData(3)
# Ok, chaining callbacks is ... different.

d.addCallback(cbPrint)
# Before return result, this was printing None.  
d.addCallback(cbPrint2)
d.addCallback(cbPrint)
d.addCallback(print)

print("Stop the reactor in 7 seconds...")
reactor.callLater(7, reactor.stop)
print("starting the reactor")
reactor.run()


print("Reactor has been safely shutdown.  :P")