'When methods from the interface are called on instances of the class, a
``Deferred`` is returned which fires with the result of the call to the
corresponding method on the provider of the interface.'

def test_success_result(self):

    args = (self.a, self.b, self.c)
    result = self.successResultOf(self.async_spy.method(*args))
    self.assertEqual(self.spy.method(*args), result)
