automata.py

class DeterministicFiniteAutomaton:
  def __init__(self, initial_state, transition_table):
    self.state_old   = None
    self.state       = initial_state
    self.transitions = transition_table

  def read(self, letter):
    newstate = self.transitions[(self.state, letter)]

    self.oldstate = self.state
    self.state = newstate[0]
    
    if self.oldstate != self.state and callable(newstate[1]):
      newstate[1]()

  def read_list(self, letters):
    map(self.read, letters)

use.py

from __future__ import print_function

import RPi.GPIO as GPIO
from automata import DeterministicFiniteAutomaton as DFA

GPIO.setmode(GPIO.BCM)

led = 4
button_no = 8                   # (n)ormally (o)pen

GPIO.setup(led,       GPIO.OUT)
GPIO.setup(button_no, GPIO.IN, pull_up_down=GPIO.PUD_DOWN)

def turnon():  GPIO.output(led, GPIO.HIGH)

def turnoff(): GPIO.output(led, GPIO.LOW)

transitions = {
    ('on',     0): ('on',     None),
    ('on',     1): ('on-off', None),
    ('on-off', 1): ('on-off', None),
    ('on-off', 0): ('off',    turnoff),
    ('off',    0): ('off',    None),
    ('off',    1): ('off-on', None),
    ('off-on', 1): ('off-on', None),
    ('off-on', 0): ('on',     turnon)
}

onoff = DFA('off', transitions)

import time
try:
    GPIO.output(led, GPIO.LOW)
    while True:
        onoff.read(GPIO.input(button_no))
        time.sleep(.001)
except KeyboardInterrupt:
    GPIO.cleanup()