Effect-inspired library idea aimed at unit testing Deferred-using code

gistfile1.py

# my_effect.py
"""
In normal code, side-effecty code is run as usual.

In tests one can (with a context manager, perhaps) disable side-effects and figure out 
what the intention of the code is without actually triggering any side-effects.
"""

class IEffect(Interface):
     def run():
       "return a Deferred that will eventually fire"

IN_TEST_MODE = False

def result_of(effect):
    if IN_TEST_MODE:
        d = Deferred()
        effect.deferred = d
        d.__effect__ = effect
        return d
    else:
        return effect.run()

def get_intended_effect(d):
    """For use with unit tests only."""
    return d.__effect__

@contextmanager
def test_effects():
    """For use with unit tests only."""
    global IN_TEST_MODE
    IN_TEST_MODE = True
    try:
        yield
    finally:
        IN_TEST_MODE = False

def resolve_effect_to(effect, result):
    """For use with unit tests only."""
    effect.deferred.callback(result)


# ============ Example usage: ========

from myeffect import result_of, get_intended_effect, resovle_effect_to

def mycode():
    d = result_of(MyEffect(123))
    d.addCallback(blah)
    return d


def test_mycode():
    with test_effects:
        effect = get_intended_effect(mycode())
        self.assertEqual(effect, MyEffect(123))

    with test_effects:
        d = mycode()
        resolve_effect_to(d, 456)
        self.assertEqual(self.successResultOf, blah(456))