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functional tools for cooperative multitasking.
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cotools is a collection of functional programming primitives that make use of the cooperative multitasking facilities found in Twisted.

The goal is to provide a more comfortable mechanism for breaking up and cooperatively scheduling computation than those discussed in my talk Cooperative Multitasking with Twisted (video) where I discuss how to break up and schedule work using reactor.callLater and twisted.internet.task.coiterate. The primitives provided by cotools utilize twisted.internet.task.coiterate and also attempt to be fully Deferred aware. Meaning that Deferreds are valid values as iterators, in iterators, and as return values of the functions to be applied to the items in the iterators.

I admit that these examples are rather contrived and the implementation though having significant test coverage may not be optimal or performant. My goal is more to facilitate discussion about how to make Cooperative Multitasking a more comfortable programming paradigm and to attempt to give novice Twisted users some level of cooperation 'for free'.

Non-blocking vs Cooperative

These primitives much like the Deferred they are built on do not magically make your code non-blocking. The primary unit of work is the item in the iterator and whatever function does with that item may block your application for any period of time.


All primitives return a Deferred which will callback when iteration is completed. For the purposes of brevity in the documentation "returns" always means "returns a Deferred that callbacks with" unless otherwise noted.

  • coforeach(function, iterator) - Applies function to each item in iterator.
  • comap(function, iterator) - Applies function to each item in iterator returning a list of the return values of function(item).
  • cofilter(function, iterator) - Return items in iterator for which function(item) returns True.
  • cofold(funciton, accumulator, iterator) - Calls function(accumulator, item) for each item in iterator using the return value as the accumulator for the next item. cofold will return the last accumulator.
  • cotakewhile(function, iterator) - Take items found in iterator and return them until an item is found where function(item) does not return true.
  • cosum(iterator) - Sum all the items in iterator. Implemented as cofold(lambda a, b: a + b, 0, iterator). More of a toy than a useful primitive.


def printr(r):
    print r

def double(x):
    return x * 2

d = comap(double, xrange(0, 5))

The above would print [0, 2, 4, 6, 8].

d = cosum(comap(double, xrange(0, 5)))

This would print 20

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