External State Storage

StateMachine can read/write state through user-provided accessor and mutator functions.

Why Use It

• State owned by a domain object
• Persistence-backed state
• Integrating with existing aggregate/root models

Example

external = {"state": State.A}

def get_state():
    return external["state"]

def set_state(new_state):
    external["state"] = new_state

sm = StateMachine[State, Trigger](
    State.A,
    state_accessor=get_state,
    state_mutator=set_state,
)

Notes

• Accessor is used as source of truth for reads.
• Mutator is called during successful state transitions.
• Exceptions raised by accessor/mutator propagate to the caller.
• If accessor initial value differs from constructor initial_state, accessor value takes precedence after initialization.