defer-cancel-12.py

from twisted.internet import defer

def cancel_outer(d):
    print("outer cancel callback.")

def cancel_inner(d):
    print("inner cancel callback.")

def first_outer_callback(res):
    print('first outer callback, returning inner deferred')
    return inner_d

def second_outer_callback(res):
    print('second outer callback got:', res)

def outer_errback(err):
    print('outer errback got:', err)

outer_d = defer.Deferred(cancel_outer)
inner_d = defer.Deferred(cancel_inner)

outer_d.addCallback(first_outer_callback)
outer_d.addCallbacks(second_outer_callback, outer_errback)

outer_d.callback('result')

# at this point the outer deferred has fired, but is paused
# on the inner deferred.

print('canceling outer deferred.')
outer_d.cancel()

print('done')