A simple example of a state machine

simple_state_machine.py

from __future__ import print_function
import sys
from collections import namedtuple
from pprint import pprint

STATE_CONST = (
    'STOPPED',
    'STARTING',
    'RUNNING',
    'STOPPING',
    'ABORTED',
    )
STATES = {}

i = 0
m = sys.modules[__name__]
for s in STATE_CONST:
    v = 1 << i
    STATES[s] = v
    setattr(m, s, v)
    i += 1

State = type('State', (), STATES)
STATES.update({v: k for k, v in STATES.items()})

class StateMachine(object):

    def __init__(self, state=None):
        if state is None:
            self.state = State.STOPPED
        elif state in STATES and state != State.ABORTED:
            self.state = state
        else:
            self.state = State.ABORTED

    def log(self, to_state):
        print('Changing from {0} to {1}'.format(STATES[self.state], STATES[to_state]))

    def error(self, to_state):
        print('ERROR: Can not change from {0} to {1}'.format(STATES[self.state], STATES[to_state]))

    def control(self, to_state):
        if self.state == to_state:
            self.error(to_state)
        elif self.state == State.STOPPED:
            if to_state == State.STARTING:
                self.log(to_state)
                self.state = State.STARTING
            else:
                self.error(to_state)
        elif self.state == State.STARTING:
            if to_state == State.RUNNING:
                self.log(to_state)
                self.state = State.RUNNING
            elif to_state == State.ABORTED:
                self.log(to_state)
                self.state = State.ABORTED
            else:
                self.error(to_state)
        elif self.state == State.RUNNING:
            if to_state == State.STOPPING:
                self.log(to_state)
                self.state = State.STOPPING
            elif to_state == State.ABORTED:
                self.log(to_state)
                self.state = State.ABORTED
            else:
                self.error(to_state)
        elif self.state == State.STOPPING:
            if to_state == State.STOPPED:
                self.log(to_state)
                self.state = State.STOPPED
            elif to_state == State.ABORTED:
                self.log(to_state)
                self.state = State.ABORTED
            else:
                self.error(to_state)
        else:
            self.error(to_state)


if __name__ == '__main__':
    m = StateMachine(State.STARTING)
    m.control(State.STARTING)
    m.control(State.RUNNING)
    m.control(State.STOPPING)
    m.control(State.ABORTED)
    m.control(State.STOPPED)