Reusing transitions

This example helps to turn visual the different compositions of how to declare and bind Transitions to Event.

Note

Even sharing the same transition instance, only the transition actions associated with the event will be called.

TrafficLightMachine

The same transitions are bound to more than one event.

TrafficLightIsolatedTransitions

We define new transitions, thus, isolating the connection between states.

from statemachine import State
from statemachine import StateMachine


class TrafficLightMachine(StateMachine):
    "A traffic light machine"

    green = State(initial=True)
    yellow = State()
    red = State()

    slowdown = green.to(yellow)
    stop = yellow.to(red)
    go = red.to(green)

    cycle = slowdown | stop | go

    def before_slowdown(self):
        print("Slowdown")

    def before_cycle(self, event: str, source: State, target: State, message: str = ""):
        message = ". " + message if message else ""
        return f"Running {event} from {source.id} to {target.id}{message}"

    def on_enter_red(self):
        print("Don't move.")

    def on_exit_red(self):
        print("Go ahead!")

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Run a transition

sm = TrafficLightMachine()
sm.send("cycle")

'Running cycle from green to yellow'

class TrafficLightIsolatedTransitions(StateMachine):
    "A traffic light machine"

    green = State(initial=True)
    yellow = State()
    red = State()

    slowdown = green.to(yellow)
    stop = yellow.to(red)
    go = red.to(green)

    cycle = green.to(yellow) | yellow.to(red) | red.to(green)

    def before_slowdown(self):
        print("Slowdown")

    def before_cycle(self, event: str, source: State, target: State, message: str = ""):
        message = ". " + message if message else ""
        return f"Running {event} from {source.id} to {target.id}{message}"

    def on_enter_red(self):
        print("Don't move.")

    def on_exit_red(self):
        print("Go ahead!")

Run a transition

sm2 = TrafficLightIsolatedTransitions()
sm2.send("cycle")

'Running cycle from green to yellow'