[Twisted-Python] async code structure question

[Mary Gardiner]

On Wed, Feb 09, 2005, snacktime wrote:
> Say I have a class with several functions that are called in order. 
> Each function relies on the results of the previous function in order
> to proceed.  To write this asynchronously, do I chain them together so
> that each function is called as a deferred, with the callback pointing
> to the next function and so forth down the list?

It depends on whether these are synchronous or asynchonrous functions.

For synchronous, let's assume you call one function that returns a
Deferred, and then the rest of them manipulate the results of that
function. You just add all the manipulating functions as callbacks:

from twisted.internet import defer

def getDummyDeferred():
    """
    Dummy method which returns a Deferred for us to play with
    Usually this would be some library function that does something async
    """
    # defer.succeed returns a Deferred fired with its argument
    return defer.succeed(3)

def multiplyByTwo(result):
    return result * 2

def square(result):
    return result * result

def printResult(result):
    print "result is %d" % result

def main():
    d = getDummyDeferred()
    d.addCallback(multiplyByTwo)
    d.addCallback(square)
    d.addCallback(printResult)

if __name__ == '__main__':
    main()

For asynchronous, you chain the Deferreds:

from twisted.internet import defer

def getDummyDeferred():
    return defer.succeed(2)

def getDummyDeferredTwo(result):
    return defer.succeed(result * 3)

def printResult(result):
    print result

def main():
    d = getDummyDeferred()
    d.addCallback(getDummyDeferredTwo)
    d.addCallback(printResult)

You could have some fun converting these all to use reactor.callLater in
the getDummyDeferred and getDummyDeferredTwo methods rather than using
defer.succeed to generate an already fired Deferred :) (This will mean
using reactor.run() in main.)

Relevant examples are at
http://twistedmatrix.com/documents/current/howto/defer#auto5

For (a tiny bit) more on chaining Deferreds together, have a look at
http://twistedmatrix.com/documents/current/howto/defer#auto12. There's
already a bug filed about improving this bit:
http://twistedmatrix.com/bugs/issue596

Comments welcome.

-Mary