ro-fsm 0.6.0

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Ro FSM Library

A lightweight, context-driven finite state machine (FSM) library for Python that supports:

• States with entry and exit actions.
• Transitions with conditions based on a user-defined context.
• Wildcard transitions for global state changes.
• YAML-based configuration for declarative FSM definitions.
• Safe evaluation of actions and conditions (only allowing context-based access).

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Installation

pip install ro_fsm

Requires Python ≥3.8 and PyYAML for YAML parsing.

Overview

A state machine consists of:

• States – Named entities with optional entry/exit actions.
• Transitions – Connections between states, each with a boolean condition evaluated against a context object.
• Context – User-defined object containing the data and methods that drive FSM logic.

Python API

State

Represents a single state in the FSM.

State(name: str, entry_action: Optional[Callable[[Any], None]] = None,
      exit_action: Optional[Callable[[Any], None]] = None)

• name – Unique string identifier for the state.
• entry_action – Function called when the state is entered (receives context).
• exit_action – Function called when the state is exited (receives context).

Example:

idle = State(
    "Idle",
    entry_action=lambda ctx: print("Entering Idle"),
    exit_action=lambda ctx: print("Exiting Idle")
)

Transition

Encapsulates a transition to another state.

Transition(to_state: str, condition: Optional[Callable[[Any], bool]] = None)

• to_state – Destination state name.
• condition – Function returning a boolean; triggers transition if True.

StateMachine

Manages states, transitions, and context-driven updates.

StateMachine(initial_state: State, context: Any = None, s0_action_flag: bool = True)

• initial_state – Starting state of the FSM.
• context – Object passed to all actions and conditions.
• s0_action_flag – Whether to run the entry action of the initial state immediately.

Methods

• add_state(state: State) – Add a new state.
• add_transition(from_state: str, to_state: str, condition: Callable[[Any], bool]) – Add a transition between states.
• set_state(state_name: str, context: Any) – Force a state change.
• update(context: Any = None) – Evaluate transitions and update the state.

YAML Constructor

StateMachine.from_yaml(path: str, context: Any, s0_action_flag: bool = True)

• Loads states, transitions, and initial state from a YAML file.
• Wildcard transitions (from: "*") have priority over state-specific transitions.

YAML Configuration

Define your FSM declaratively:

initial_state: Idle

states:
  - name: Idle
    entry_action: context.log("Idle state")
    exit_action: context.cleanup()
    transitions:
      - to: Active
        condition: context.ready()

  - name: Active
    entry_action: context.start()
    transitions:
      - to: Finished
        condition: context.done()

  - name: Finished

# Wildcard transitions apply globally
transitions:
  - from: "*"
    to: OutOfOrder
    condition: context.error

Rules:

1. States cannot be named "*".
2. Actions and conditions must only reference context.
3. Top-level transitions (wildcards) override state-specific transitions.
4. Expression evaluation is sandboxed via AST parsing to prevent unsafe code execution.

Example Usage

from fsm import StateMachine
from context import Context  # user-defined context class

ctx = Context()
sm = StateMachine.from_yaml("vending_machine.yaml", ctx)

# Initial state
print(sm.current_state.name)

# Step through FSM
while sm.current_state.name != "Finished":
    sm.update(ctx)

Best Practices

• Keep actions and conditions pure: avoid side effects outside the context object.
• Use wildcard transitions for emergency/error handling.
• Validate YAML using schema tools to prevent misconfigurations.
• Use entry_action and exit_action for logging, resource management, or triggering side effects.

Safety & Security

• Conditions and actions are evaluated in a sandbox.
• Only context and its attributes/methods can be accessed in YAML expressions.
• AST parsing prevents arbitrary code execution.

License

MIT License.