'Attaches a callable that will be called when the future finishes.
Args:
fn: A callable that will be called with this future as its only
argument when the future completes or is cancelled. The callable
will always be called by a thread in the same process in which
it was added. If the future has already completed or been
cancelled then the callable will be called immediately. These
callables are called in the order that they were added.'

def add_done_callback(self, fn):

    with self._condition:
        if (self._state not in [CANCELLED, CANCELLED_AND_NOTIFIED, FINISHED]):
            self._done_callbacks.append(fn)
            return
    fn(self)
