pystate 0.1.1

Python package for co-routine base state machines.

FiniteStateMachine - A finite state machine class using coroutines

FiniteStateMachine is a class representing a finite state machine. Each state is represented by an instance of the
State class. Each state also has a state handler function defined for it. The handler function is a co-routine that
excepts an event and performs an action based on it.

To define the state machine:

1) define the states (e.g. STATE_A = State('STATE_A'))
2) define the state handler functions. See below for the structure of a state handling function.
3) create an instance of the state machine (e.g. fsm = Fsm())
4) add states to the state machine, including exactly one state marked as the initial state. Each state also takes
a sequence of states that it can be transitioned from (from_states).
5) call the start function on the FSM (e.g. fsm.start())

For each event you will need to call the dispatch_event function (e.g. fsm.dispatch_event()) to route the event
to the co-routine. An event can be anything you want (e.g. a tuple with event_id and arguments). The main loops
generally looks like:

try:
while True:
event = get_next_event()
fsm.dispatch_event(event)
except ExpectedExit as e:
pass

The basic structure of a state handler is:

def state_handler_<state name>(fsm):
# Enter the main loop for the co-routine
while True:
event = yield

if event == 'EVENT_1':
# Transition to another state
fsm.transition_to(STATE_X)
elif event == 'EVENT_2':
# Do some processing but stay in this state
print('Got EVENT_2')
elif event == 'TERMINATING_EVENT':
raise FsmExit
else:
print('Unrecognized event (%s)' % event)

A simple example of this is shown in the turnstile_test.py test case.

For convenience this can be wrapped with a @state_handler decorator. The decorator takes care of the co-routine
boiler plate and hands the handler function an fsm and event. This would look like:

@pystate.state_handler
def state_locked_handler(event, fsm):
if event == 'EVENT_1':
# Transition to another state
fsm.transition_to(STATE_X)
elif event == 'EVENT_2':
# Do some processing but stay in this state
print('Got EVENT_2')
elif event == 'TERMINATING_EVENT':
raise FsmExit
else:
print('Unrecognized event (%s)' % event)

There are two ways to handle a state that needs to keep persistant data. You can create a callable clas (i.e. define
the __call__ dunder method to call as the state handler.) This allows you to use the state_handler decorator around
the __call__ method. Alternatively, you can set the state data above the while loop if you define the co-routine by
hand, however, this precludes using the decorator. See the callable_test.py test case for an example.

Author: Len Wanger
Last Updated: 7/7/2016
Copyright (c) 2016 Len Wanger

Permission is hereby granted, free of charge, to any person obtaining a copy
of this software and associated documentation files (the "Software"), to deal
in the Software without restriction, including without limitation the rights
to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
copies of the Software, and to permit persons to whom the Software is
furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all
copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
SOFTWARE.

Note: Substantial code was adapted from Christian Maugg's pystatemachine code
Copyright (c) 2015 Christian Maugg
(https://raw.githubusercontent.com/cmaugg/pystatemachine/master/pystatemachine.py)