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A Finite Transactional State Machine.

Project description

Finite Transactional State Machine

Finite Transactional State Machine is a Transaction driven finite state machine. Transaction can be any Python callable object that is reverted when exceptions occur.

Installation

pip3 install ftsm

How does it work ?

  1. Create states and list of possible transitions the state is allowed to transition to.
    UNLOCKED = State('UNLOCKED', initial=True, allowed_transitions=['LOCKED'])
    LOCKED = State('LOCKED', initial=False, allowed_transitions=['UNLOCKED'])
    
  2. Initialize the transitional state machine.
    tsm = TransactionalFiniteStateMachine(name='Lock')
    
  3. Add defined states to a state machine.
    tsm.add(LOCKED)
    tsm.add(UNLOCKED)
    
  4. Create transaction and define rollback transactions with or without conditions.
    t1 = Transaction(
    target=func,
    args=('name',),
    rb_transactions=[t2],
    rb_conditions=[ExceptionCondition(KeyError)])
    
  5. Transition to a new state with transactions.
    with tsm.managed_transition(
            state=LOCKED,
            pre_transactions=[t1, t3],
            on_error_transactions=[t4],
            post_transactions=[t5]):
        func()
    

Example

from ftsm import State, Transaction, TransactionalFiniteStateMachine

class LightController:
    def turn_off_light(self, room):
        print('turning the {} room light off.'.format(room))

    def turn_on_light(self, room):
        print('turning the {} room light on.'.format(room))

light_controller = LightController()

def turn_off_water():
    print('turning off the water.')

def turn_on_water():
    print('turning on the water.')

def water_plants():
    print('watering the plants.')

def lock_the_door():
    print('locking the door.')

def unlock_the_door():
    print('unlocking the door.')

UNLOCKED = State('UNLOCKED', initial=True, allowed_transitions=['LOCKED'])
LOCKED = State('LOCKED', initial=False, allowed_transitions=['UNLOCKED'])

tsm = TransactionalFiniteStateMachine(name='Lock')
tsm.add(LOCKED)
tsm.add(UNLOCKED)

light_transaction = Transaction(
    target=light_controller.turn_off_light,
    args=('Living',),
    rb_transactions=[
        Transaction(target=light_controller.turn_on_light,
                    args=('Living',))
    ])

water_transaction = Transaction(
    target=turn_off_water,
    rb_transactions=[
        Transaction(target=turn_on_water)
    ]
)

with tsm.managed_transition(
        state=LOCKED,
        pre_transactions=[light_transaction, water_transaction],
        on_error_transactions=[Transaction(unlock_the_door)],
        post_transactions=[Transaction(water_plants)]):
    lock_the_door()

print(tsm.current_state)

Above sample code would result in following output.

turning the Living room light off.
turning off the water.
locking the door.
watering the plants.
<State name=LOCKED initial=False>

If errors occur while performing the transactions, revert transactions are performed in the reverse order and state transition does not happens.

Rollback transaction can also be made conditional using the ExceptionCondition class provided.

light_transaction = Transaction(
    target=light_controller.turn_off_light,
    args=('Living',),
    rb_transactions=[
        Transaction(target=light_controller.turn_on_light,
                    args=('Living',))
    ],
    rb_conditions=[ExceptionCondition(KeyError)])

Above transaction now only be reverted if KeyError is encountered during the transaction execution.

User can extend the abstract Condition class to defined new Condition.

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