Feat: add a global finalizer callback

After #184 has been merged, this library would have a global preparation function but not a corresponding global finalizer that would fire once, both for successful/failed transitions, something necessary when implementing protocols like 2-phase commits. I believe that there is no way to workaround such functionality with code external to this library in a single place (i.e. without proliferating try...except/finally in all trigger-calls).

Therefore I suggest to add a finalize_after_all_transitions callback in a finally block inside Event._trigger(). The code for such a feature, without error handling would be something like this:

        for func in self.machine.prepare_conditions_check:
            self.machine._callback(func, event_data)
            logger.debug("Executed machine preparation callback '%s' before conditions." % func)
        ok = False
        try:
            for t in self.transitions[state.name]:
                event_data.transition = t
                if t.execute(event_data):
                    return True
            return False
        finally:
            event_data.kwargs['ok'] = ok  # Provisional name for `ok` flag, or an attr on machine?
            for func in self.machine.finalize_after_all_transitions:
                self.machine._callback(func, event_data)
                logger.debug("Executed machine finalize callback '%s' after all transitions." % func)

Of course if finalize cb is to receive the error raised, an except case is need additionally in the try block to store the captured exception. Like the ok flag above, I’m thinking of 2 possibilities where to store it:

a) on the machine as an extra attribute, or b) on the event-data, as another keyword item e.g. error.

Case (a) needs clearing this attribute before every trigger and kind of violate separation-of-concerns law; case (b) might clash with names already used by existing clients of the library.

[ankostis]

Yes, that would be the a perfect solution. Thanks.

[aleneum]

resolved by #175 I hope