teach_me_how_to_twisted.py

from twisted.internet import reactor
from twisted.internet.task import Clock, deferLater
from twisted.internet.defer import succeed


def opaque_animal_noise_generator():
    """
    Opaque piece of test infra that we can't touch (actually does networking or
    something so really uses the real reactor).
    """
    return deferLater(reactor, 0.5, lambda: u'meow')


def code_under_test(dependency, r):
    d = opaque_animal_noise_generator()

    def dependency_loop(noise):
        a = dependency(noise)

        def we_done(arg):
            if arg == "not_yet":
                return deferLater(r, 1000.0, dependency_loop, noise)
            else:
                return succeed(arg + ' for everyone!')
        a.addCallback(we_done)
        return a

    d.addCallback(dependency_loop)
    return d


def test():
    r = Clock()
    answers = iter(['not_yet', 'not_yet', 'Money'])

    def test_func(_):
        # Ew? Maybe? have to callLater since code under test needs
        # us to return before it puts things on the Clock.
        reactor.callLater(0.0, lambda: r.advance(1000.0))
        return succeed(next(answers))

    d = code_under_test(test_func, r)

    def verify(o):
        if o != 'Money for everyone!':
            print "FAIL!"
    d.addCallback(verify)
    return d

d = test()
d.addCallback(lambda _: reactor.stop())
reactor.run()

Solution #1 that both Richard and Tom come up with separately: don't have opaque dependencies in your testing infrastructure that use the real reactor.

That's a very good solution.