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Recursive Functional State Machine (Python recipe) by Stefan Tunsch
ActiveState Code (http://code.activestate.com/recipes/577709/)

This is a simple state machine that takes a functional approach. It requires trampoline from pysistence.func to avoid the recursion limit.

Namedtuples are used to define the different states. globals() is used to reference the states. (This could also be done putting states into a separate module and getting them through getattr.)

In this recipe the functions called in the different states need to return a boolean, which defines the success or failure event.

Python, 38 lines
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import collections
from functools import partial
from pysistence.func import trampoline

def setup():
    print 'Setting up...'
    return True
def first_step():
    print 'First step...'
    return True
def second_step():
    print 'Second step...'
    return True
def wait():
    print 'Waiting...'
    return True
def stop():
    print 'Stopping...'
    return True

State = collections.namedtuple('State', 'name function on_success on_failure')

do_start = State(name='start', function=setup, on_success='do_first_step', on_failure='do_stop')
do_first_step = State(name='first_step', function=first_step, on_success='do_second_step', on_failure='do_stop')
do_second_step = State(name='second_step', function=second_step, on_success='do_wait', on_failure='do_stop')
do_wait = State(name='wait', function=wait, on_success='do_first_step', on_failure='do_stop')
do_stop = State(name='stop', function=stop, on_success=None, on_failure=None)

def state_machine(state):
    if state.function():
        new_state = globals().get(state.on_success, None)
    else:
        new_state = globals().get(state.on_failure, None)
    if new_state:
        return partial(state_machine, new_state)

if __name__ == '__main__':        
    trampoline(state_machine, do_start)
Tags: functional, recursion, state_machine