python-statemachine 1.0.0

Python Finite State Machines made easy.

Python finite-state machines made easy.

• Free software: MIT license

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

Welcome to python-statemachine, an intuitive and powerful state machine framework designed for a great developer experience.

🚀 With StateMachine, you can easily create complex, dynamic systems with clean, readable code.

💡 Our framework makes it easy to understand and reason about the different states, events and transitions in your system, so you can focus on building great products.

🔒 python-statemachine also provides robust error handling and ensures that your system stays in a valid state at all times.

A few reasons why you may consider using it:

• 📈 python-statemachine is designed to help you build scalable, maintainable systems that can handle any complexity.

• 💪 You can easily create and manage multiple state machines within a single application.

• 🚫 Prevents common mistakes and ensures that your system stays in a valid state at all times.

Getting started

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

$ pip install python-statemachine

To generate diagrams from your machines, you’ll also need pydot and Graphviz. You can install this library already with pydot dependency using the extras install option. See our docs for more details.

$ pip install python-statemachine[diagrams]

Define your state machine:

>>> 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)
...
...     slowdown = green.to(yellow)
...     stop = yellow.to(red)
...     go = red.to(green)
...
...     def before_cycle(self, event_data=None):
...         message = event_data.kwargs.get("message", "")
...         message = ". " + message if message else ""
...         return "Running {} from {} to {}{}".format(
...             event_data.event,
...             event_data.transition.source.id,
...             event_data.transition.target.id,
...             message,
...         )
...
...     def on_enter_red(self):
...         print("Don't move.")
...
...     def on_exit_red(self):
...         print("Go ahead!")

You can now create an instance:

>>> traffic_light = TrafficLightMachine()

Then start sending events:

>>> traffic_light.cycle()
'Running cycle from green to yellow'

You can inspect about the current state:

>>> traffic_light.current_state.id
'yellow'

Or get a complete state repr for debugging purposes:

>>> traffic_light.current_state
State('Yellow', id='yellow', value='yellow', initial=False, final=False)

The State instance can also be checked by equality:

>>> traffic_light.current_state == TrafficLightMachine.yellow
True

>>> traffic_light.current_state == traffic_light.yellow
True

But for your convenience, can easily ask if a state is active at any time:

>>> traffic_light.green.is_active
False

>>> traffic_light.yellow.is_active
True

>>> traffic_light.red.is_active
False

Easily iterate over all states:

>>> [s.id for s in traffic_light.states]
['green', 'red', 'yellow']

Or over events:

>>> [t.name for t in traffic_light.events]
['cycle', 'go', 'slowdown', 'stop']

Call an event by it’s name:

>>> traffic_light.cycle()
Don't move.
'Running cycle from yellow to red'

Or sending an event with the event name:

>>> traffic_light.send('cycle')
Go ahead!
'Running cycle from red to green'

>>> traffic_light.green.is_active
True

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

>>> traffic_light.go()
Traceback (most recent call last):
statemachine.exceptions.TransitionNotAllowed: Can't go when in Green.

Keeping the same state as expected:

>>> traffic_light.green.is_active
True

And you can pass arbitrary positional or keyword arguments to the event, and they will be propagated to all actions and callbacks:

>>> traffic_light.cycle(message="Please, now slowdon.")
'Running cycle from green to yellow. Please, now slowdon.'

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.red.is_active
True

>>> obj.state
'red'

>>> obj.state = 'green'

>>> traffic_light.green.is_active
True

>>> traffic_light.slowdown()

>>> obj.state
'yellow'

>>> traffic_light.yellow.is_active
True

A more useful example

A simple didactic state machine for controlling an Order:

>>> class OrderControl(StateMachine):
...     waiting_for_payment = State("Waiting for payment", initial=True)
...     processing = State("Processing")
...     shipping = State("Shipping")
...     completed = State("Completed", final=True)
...
...     add_to_order = waiting_for_payment.to(waiting_for_payment)
...     receive_payment = (
...         waiting_for_payment.to(processing, cond="payments_enough")
...         | waiting_for_payment.to(waiting_for_payment, unless="payments_enough")
...     )
...     process_order = processing.to(shipping, cond="payment_received")
...     ship_order = shipping.to(completed)
...
...     def __init__(self):
...         self.order_total = 0
...         self.payments = []
...         self.payment_received = False
...         super(OrderControl, self).__init__()
...
...     def payments_enough(self, amount):
...         return sum(self.payments) + amount >= self.order_total
...
...     def before_add_to_order(self, amount):
...         self.order_total += amount
...         return self.order_total
...
...     def before_receive_payment(self, amount):
...         self.payments.append(amount)
...         return self.payments
...
...     def after_receive_payment(self):
...         self.payment_received = True
...
...     def on_enter_waiting_for_payment(self):
...         self.payment_received = False

You can use this machine as follows.

>>> control = OrderControl()

>>> control.add_to_order(3)
3

>>> control.add_to_order(7)
10

>>> control.receive_payment(4)
[4]

>>> control.current_state.id
'waiting_for_payment'

>>> control.process_order()
Traceback (most recent call last):
...
statemachine.exceptions.TransitionNotAllowed: Can't process_order when in Waiting for payment.

>>> control.receive_payment(6)
[4, 6]

>>> control.current_state.id
'processing'

>>> control.process_order()

>>> control.ship_order()

>>> control.payment_received
True

>>> control.order_total
10

>>> control.payments
[4, 6]

>>> control.completed.is_active
True

There’s a lot more to cover, please take a look at our docs: https://python-statemachine.readthedocs.io.