python-statemachine 0.8.0

Python Finite State Machines made easy.

Python finite-state machines made easy.

• Free software: MIT license

• Documentation: https://python-statemachine.readthedocs.io.

Getting started

To install Python State Machine, run this command in your terminal:

$ pip install python-statemachine

Define your state machine:

from statemachine import StateMachine, State

class TrafficLightMachine(StateMachine):
    green = State('Green', initial=True)
    yellow = State('Yellow')
    red = State('Red')

    slowdown = green.to(yellow)
    stop = yellow.to(red)
    go = red.to(green)

You can now create an instance:

>>> traffic_light = TrafficLightMachine()

And inspect about the current state:

>>> traffic_light.current_state
State('Green', identifier='green', value='green', initial=True)
>>> traffic_light.current_state == TrafficLightMachine.green == traffic_light.green
True

For each state, there’s a dynamically created property in the form is_<state.identifier>, that returns True if the current status matches the query:

>>> traffic_light.is_green
True
>>> traffic_light.is_yellow
False
>>> traffic_light.is_red
False

Query about metadata:

>>> [s.identifier for s in m.states]
['green', 'red', 'yellow']
>>> [t.identifier for t in m.transitions]
['go', 'slowdown', 'stop']

Call a transition:

>>> traffic_light.slowdown()

And check for the current status:

>>> traffic_light.current_state
State('Yellow', identifier='yellow', value='yellow', initial=False)
>>> traffic_light.is_yellow
True

You can’t run a transition from an invalid state:

>>> traffic_light.is_yellow
True
>>> traffic_light.slowdown()
Traceback (most recent call last):
...
TransitionNotAllowed: Can't slowdown when in Yellow.

You can also trigger events in an alternative way, calling the run(<transition.identificer>) method:

>>> traffic_light.is_yellow
True
>>> traffic_light.run('stop')
>>> traffic_light.is_red
True

A state machine can be instantiated with an initial value:

>>> machine = TrafficLightMachine(start_value='red')
>>> traffic_light.is_red
True

Models

If you need to persist the current state on another object, or you’re using the state machine to control the flow of another object, you can pass this object to the StateMachine constructor:

>>> class MyModel(object):
...     def __init__(self, state):
...         self.state = state
...
>>> obj = MyModel(state='red')
>>> traffic_light = TrafficLightMachine(obj)
>>> traffic_light.is_red
True
>>> obj.state
'red'
>>> obj.state = 'green'
>>> traffic_light.is_green
True
>>> traffic_light.slowdown()
>>> obj.state
'yellow'
>>> traffic_light.is_yellow
True

Callbacks

Callbacks when running events:

from statemachine import StateMachine, State

class TrafficLightMachine(StateMachine):
    "A traffic light machine"
    green = State('Green', initial=True)
    yellow = State('Yellow')
    red = State('Red')

    slowdown = green.to(yellow)
    stop = yellow.to(red)
    go = red.to(green)

    def on_slowdown(self):
        print('Calma, lá!')

    def on_stop(self):
        print('Parou.')

    def on_go(self):
        print('Valendo!')

>>> stm = TrafficLightMachine()
>>> stm.slowdown()
Calma, lá!
>>> stm.stop()
Parou.
>>> stm.go()
Valendo!

Or when entering/exiting states:

from statemachine import StateMachine, State

class TrafficLightMachine(StateMachine):
    "A traffic light machine"
    green = State('Green', initial=True)
    yellow = State('Yellow')
    red = State('Red')

    cycle = green.to(yellow) | yellow.to(red) | red.to(green)

    def on_enter_green(self):
        print('Valendo!')

    def on_enter_yellow(self):
        print('Calma, lá!')

    def on_enter_red(self):
        print('Parou.')

>>> stm = TrafficLightMachine()
>>> stm.cycle()
Calma, lá!
>>> stm.cycle()
Parou.
>>> stm.cycle()
Valendo!

Mixins

Your model can inherited from a custom mixin to auto-instantiate a state machine.

class CampaignMachineWithKeys(StateMachine):
    "A workflow machine"
    draft = State('Draft', initial=True, value=1)
    producing = State('Being produced', value=2)
    closed = State('Closed', value=3)
    cancelled = State('Cancelled', value=4)

    add_job = draft.to.itself() | producing.to.itself()
    produce = draft.to(producing)
    deliver = producing.to(closed)
    cancel = cancelled.from_(draft, producing)


class MyModel(MachineMixin):
    state_machine_name = 'CampaignMachineWithKeys'

    def __init__(self, **kwargs):
        for k, v in kwargs.items():
            setattr(self, k, v)
        super(MyModel, self).__init__()

    def __repr__(self):
        return "{}({!r})".format(type(self).__name__, self.__dict__)


model = MyModel(state='draft')
assert isinstance(model.statemachine, campaign_machine)
assert model.state == 'draft'
assert model.statemachine.current_state == model.statemachine.draft

model.statemachine.cancel()
assert model.state == 'cancelled'

History

0.8.0 (2020-01-23)

• Add support for Python 3.7 and 3.8 (adding to test matrix).

• Update development requirements.

• State machine names should now be fully qualified for mixins, simple names are deprecated and will no longer be supported on a future version.

• Development: Adding mypy linter.

• Add support for State machine inheritance. Thanks @rschrader.

• Add support for reverse transitions: transition = state_a.from_(state_b). Thanks @romulorosa.

• Fix current state equal to destination on enter events. Thanks @robnils and @joshuacc1.

Breaking changes:

• Drop official support for Python 3.4 (removing from test matrix, code may still work).

0.7.1 (2019-01-18)

• Fix Django integration for registry loading statemachine modules on Django1.7+.

0.7.0 (2018-04-01)

• New event callbacks: on_enter_<state> and on_exit_<state>.

0.6.2 (2017-08-25)

• Fix README.

0.6.1 (2017-08-25)

• Fix deploy issues.

0.6.0 (2017-08-25)

• Auto-discovering statemachine/statemachines under a Django project when they are requested using the mixin/registry feature.

0.5.1 (2017-07-24)

• Fix bug on CombinedTransition._can_run not allowing transitions to run if there are more than two transitions combined.

0.5.0 (2017-07-13)

• Custom exceptions.

• Duplicated definition of on_execute callback is not allowed.

• Fix bug on StateMachine.on_<transition.identifier> being called with extra self param.

0.4.2 (2017-07-10)

• Python 3.6 support.

• Drop official support for Python 3.3.

• Transition can be used as decorator for on_execute callback definition.

• Transition can point to multiple destination states.

0.3.0 (2017-03-22)

• README getting started section.

• Tests to state machine without model.

0.2.0 (2017-03-22)

• State can hold a value that will be assigned to the model as the state value.

• Travis-CI integration.

• RTD integration.

0.1.0 (2017-03-21)

• First release on PyPI.