4.2. State Machine¶

StateMachine imposes a structure to automatically change the implementation from one object to the next
The current implementation represents the state that a system is in
System behaves differently from one state to the next
The code that moves the system from one state to the next
Each state can be run() to perform its behavior
You can pass it an input object so it can tell you what new state to move to based on that input
Each State object decides what other states it can move to, based on the input
Each State object has its own little State table
There is a single master state transition table for the whole system

statemachine TrafficLight:
    Red -> Amber
    Amber (if previous == Red) -> Green
    Green -> Amber
    Amber (if previous == Green) -> Red


Red.wait = sleep(2)
Amber.wait = sleep(1)
Green.wait = sleep(2)

Listing 4.119. State Machine¶

from time import sleep


class Light:
    def __init__(self, previous=None):
        self.previous = previous

    def run(self):
        raise NotImplementedError

    def __next__(self):
        raise NotImplementedError


class Red(Light):
    color = 'Red'
    wait = 2

    def run(self):
        print(self.color)
        sleep(self.wait)

    def __next__(self):
        return Amber(previous=self)


class Amber(Light):
    color = 'Amber'
    wait = 1

    def run(self):
        print(self.color)
        sleep(self.wait)

    def __next__(self):
        if isinstance(self.previous, Red):
            return Green(previous=self)
        else:
            return Red(previous=self)


class Green(Light):
    color = 'Green'
    wait = 2

    def run(self):
        print(self.color)
        sleep(self.wait)

    def __next__(self):
        return Amber(previous=self)


class TrafficLights:
    def __init__(self, initial_state=Green(), max_changes=10):
        self.state = initial_state
        self.max_changes = max_changes

    def __iter__(self):
        self.changes = 0
        return self

    def __next__(self):
        if self.changes >= self.max_changes:
            raise StopIteration

        self.changes += 1
        self.state.run()
        self.state = next(self.state)
        return self


for light in TrafficLights(max_changes=10):
    pass