FAVink

• FAVink
  • Getting Started
  • Transition Table and Initial State
  • Events
    • Event Handler Definitions
  • Car Example
  • API Reference
    • Predefined FiniteAutomata Methods
      • FiniteAutomata.get_state(self)
      • FiniteAutomata.get_allowed_transitions(self)
      • FiniteAutomata.is_allowed(self, transition)
    • Transition Methods

Over-simple python finite automata (finite-state machine) implementation

Getting Started

To install favink use the package from the PyPI repository:

pip install favink

To add finite automata feature to the class in your code you have to inherit FiniteAutomata class and define the following members:

• transitions table transitions
• initial state init_state
• event handlers methods.

For every transition the constructor creates a dynamic method with argument mask (self, *args, **kwargs). Each method is named after the transition.

To make the transition you should call the transition method. After the method has been called it changes the instance state and invoke related event handlers. If the called transition isn't allowed for the current state InvalidTransition extension will be raised.

Transition Table and Initial State

The transitions is a dictionary where keys are transition names, values define the allowed and target states:

transitions = {
    "transition_1":
    [
        "allowed_state_1",
        "target_state_1"
    ],
    "transition_2":
    [
        [
            "allowed_state_2",
            "allowed_state_3"
        ],
        "target_state_2"
    ]
}

Initial state is defined by init_state member.

Events

Making of transition triggers the following events and invokes the related handlers (if they have been implemented in the class):

• after
• before
• on

Event Handler Definitions

For every state (for example state_name) the following methods can be defined:

def before_state_name(self, name, *args, **kwargs):
    ...


def on_state_name(self, name, origin, *args, **kwargs):
    ...


def after_state_name(self, name, *args, **kwargs):
    ...

The following arguments are passed to the handlers:

• name is a invoked transition name,
• origin (passed only to on_... handler) is a previous state name,
• *args, **kwargs are positional and keyword arguments passed to the transition method.

If after_... or before... handlers raise the exception transition is aborted.

Car Example

class Car(FiniteAutomata):
    init_state = "stopped"
    transitions = {
        "start_engine": ["stopped", "idle"],
        "stop_engine": ["idle", "stopped"],
        "forward": ["idle", "moving_forward"],
        "backward": ["idle", "moving_backward"],
        "stop": [["moving_forward", "moving_backward"], "idle"],
    }

    def on_stopped(self, transition, origin, *args, **kwargs):
        print("Engine has been stopped")

    def on_idle(self, transition, origin, *args, **kwargs):
        print("I'm not moving, but engine is on")

    def on_moving_forward(self, transition, origin, *args, **kwargs):
        print("Let's go!")

    def on_moving_backward(self, transition, origin, *args, **kwargs):
        print("Why are we retreating?")

car = Car()

car.start_engine()
car.forward()
car.stop()
car.backward()
car.stop()
car.stop_engine()

Output:

I'm not moving, but engine is on
Let's go!
I'm not moving, but engine is on
Why are we retreating?
I'm not moving, but engine is on
Engine has been stopped

API Reference

Predefined FiniteAutomata Methods

FiniteAutomata.get_state(self)

Returns the current instance state name as a string.

FiniteAutomata.get_allowed_transitions(self)

Return the list contains all transactions which are allowed for the current instance state.

FiniteAutomata.is_allowed(self, transition)

Transition Methods

Dynamically defined methods for every transition (key) in the transitions dictionary.