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StateMachine 2.4.0

November 5, 2024

What's new in 2.4.0

This release introduces powerful new features for the StateMachine library: {ref}Condition expressions and explicit definition of {ref}Events. These updates make it easier to define complex transition conditions and enhance performance, especially in workflows with nested or recursive event structures.

Python compatibility in 2.4.0

StateMachine 2.4.0 supports Python 3.7, 3.8, 3.9, 3.10, 3.11, 3.12, and 3.13.

Conditions expressions in 2.4.0

This release introduces support for conditionals with Boolean algebra. You can now use expressions like or, and, and not directly within transition conditions, simplifying the definition of complex state transitions. This allows for more flexible and readable condition setups in your state machine configurations.

Example (with a spoiler of the next highlight):

from statemachine import StateMachine, State, Event

class AnyConditionSM(StateMachine):
    start = State(initial=True)
    end = State(final=True)

    submit = Event(
        start.to(end, cond="used_money or used_credit"),
        name="finish order",
    )

    used_money: bool = False
    used_credit: bool = False

sm = AnyConditionSM()
sm.submit()
# TransitionNotAllowed: Can't finish order when in Start.

sm.used_credit = True
sm.submit()
sm.current_state.id
# 'end'
See {ref}`Condition expressions` for more details or take a look at the {ref}`sphx_glr_auto_examples_lor_machine.py` example.

Explicit event creation in 2.4.0

Now you can explicit declare {ref}Events using the {ref}event class. This allows custom naming, translations, and also helps your IDE to know that events are callable.

>>> from statemachine import StateMachine, State, Event

>>> class StartMachine(StateMachine):
...     created = State(initial=True)
...     started = State(final=True)
...
...     start = Event(created.to(started), name="Launch the machine")
...
>>> [e.id for e in StartMachine.events]
['start']
>>> [e.name for e in StartMachine.events]
['Launch the machine']
>>> StartMachine.start.name
'Launch the machine'
See {ref}`Events` for more details.

Recursive state machines (infinite loop)

We removed a note from the docs saying to avoid recursion loops. Since the {ref}StateMachine 2.0.0 release we've turned the RTC model enabled by default, allowing nested events to occour as all events are put on an internal queue before being executed.

See {ref}`sphx_glr_auto_examples_recursive_event_machine.py` for an example of an infinite loop state machine declaration using `after` action callback to call the same event over and over again.

Bugfixes in 2.4.0

  • Fixes #484 issue where nested events inside loops could leak memory by incorrectly referencing previous event_data when queuing the next event. This fix improves performance and stability in event-heavy workflows.