gistfile1.py

class State(object):
    def __init__(self, default=None):
        self.default = default

    def __get__(self, instance, owner):
        return self._value(instance)

    def _value(self, instance):
        if not hasattr(instance, '_state_value'):
            setattr(instance, '_state_value', self.default)

        return getattr(instance, '_state_value')

    def transition(self, source='*', target=None, immediate=None, check=None):
        def wrapped(func):
            transitions = {}

            sources = [source] if isinstance(source, basestring) else source
            targets = [target] if isinstance(target, basestring) else target

            for s in sources:
                transitions[s] = targets

            def try_transition(target, instance, *args, **kwargs):
                current = self._value(instance)
                succ = transitions.get(current, transitions.get('*', None))

                if not succ:
                    raise RuntimeError("Transition not allowed")

                target = check(instance) if isinstance(target, list) else target
                setattr(instance, '_state_value', target)

            def call_func(instance, *args, **kwargs): 
                if immediate:
                    try_transition(immediate, instance)

                result = func(instance, *args, **kwargs)
                try_transition(target, instance)
                return result

            return call_func

        return wrapped


class Thing(object):

    state = State(default='standby')

    def __init__(self):
        self._velocity = 0

    def get_state(self):
        return 'standby' if not self._velocity else 'moving'

    @state.transition(source='*', target=['standby', 'moving'], check=get_state)
    def set_velocity(self, velocity):
        print velocity
        self._velocity = velocity

    @state.transition(source='standby', target='moving')
    def start(self):
        self._velocity = 10

    @state.transition(source='moving', target='standby')
    def stop(self):
        self._velocity = 0


thing = Thing()
thing.start()
print thing.state

thing.stop()
print thing.state

thing.set_velocity(1)
print thing.state

thing.set_velocity(0)
print thing.state