fgmacedo / Python Statemachine
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==================== Python State Machine
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Python finite-state machines <https://en.wikipedia.org/wiki/Finite-state_machine>
_ 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:
.. code-block:: console
$ 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 traffic_light.states] ['green', 'red', 'yellow'] [t.identifier for t in traffic_light.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): statemachine.exceptions.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.
from statemachine.mixins import MachineMixin
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=1) assert isinstance(model.statemachine, CampaignMachineWithKeys) assert model.state == 1 assert model.statemachine.current_state == model.statemachine.draft model.statemachine.cancel() assert model.state == 4 assert model.statemachine.current_state == model.statemachine.cancelled