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TrafficLight

Overview of stateforward.example.traffic_light.py Module

The stateforward.example.traffic_light.py module is part of the stateforward package, which provides a framework for building state machines using the asyncio capabilities of Python. This specific module includes an example implementation of a traffic light state machine.

Features:
  • Asynchronous state machine implementation for traffic lights.
  • Custom events such as PedestrianWalkButton and CarSensor.
  • Use of timers to automatically transition between states (after function).
  • Guard functions that allow conditional transitions.

Traffic Light Example Usage

Below is a possible sequence of operations for the TrafficLight state machine:

  1. The traffic light starts in the On state with the green submachine state active.
  2. When a PedestrianWalkButton event occurs, the light transitions to the yellow submachine state.
  3. After 3 seconds (simulated by the after guard condition), the light transitions to the red submachine state.

State Machine Diagram

Here is a mermaid.js diagram that visualizes the state machine for the traffic light example:

Traffic Light

stateDiagram-v2
    direction LR
    Off: Off
    Off: entry / display(self)
    Off: exit / display(self)
    On: On
    On: entry / display(self)
    On: exit / display(self)
    [*] --> On
    On --> [*]
    On --> Off : after(1s) / guard self.flashing
    Off --> On : after(1s)

stateDiagram-v2
    direction LR
    state On {

    [*] --> green
    }

stateDiagram-v2
    direction LR

    state On {
      initial --> green
      green --> yellow: PedestrianWalkButton
      yellow --> red: after(3s)
    }

    [*] --> On
    On --> Off: power_off

Development Notes

Event Classes

  • PedestrianWalkButton - Represents the event of a pedestrian pressing the walk button.
  • CarSensor - Represents the detection of a car waiting at the traffic light.
  • OffEvent - Indicates that the traffic light has been turned off.
  • FlashingEvent - Indicates the traffic light has entered a flashing state.

Behavior Implementation

  • Signal - A state machine used to represent a traffic signal mechanism that can switch between On and Off states.
  • TrafficLight - A more complex state machine representing a traffic light system that can handle pedestrian interaction and power-off events.

Guard Functions

  • walk_guard - A guard function to allow the Pedestrian region to transition from DontWalk to Walk.

Helper Functions

  • display - A debugging function to visually display the traffic light's current state.

Usage

To run the traffic light simulation, execute traffic_light_main() within an asyncio event loop, asynchronously starting the traffic light state machine and allowing for interaction through event dispatching.

Concurrency

This example takes advantage of the ConcurrencyKind.asynchronous to ensure that the traffic light state machine runs in an asynchronous environment suitable for IO-bound and high-level structured network code.