pyeds 0.7.5

Python Event Driven System

Python Event Driven System

Table of contents

• Introduction

• Installation

• How to use it

  • Declaring a FSM class

  • Declaring a state class

  • Instantiating the FSM

• Blinky example

• Event

  • Generate an event

  • Event class attributes and methods

  • Rules about event naming

• State

  • State attributes and methods

• State machine

• State transition

• Hierarchical Finite State Machines (HFSM)

• Source

Introduction

This package provides a system allows to efficiently write finite state machines (FSM) by hand. The focus was to make the API as simplest as possible since no GUI tools are included to define a FSM.

Installation

PyEDS can be installed using the standard Python tool pip with

pip install pyeds

How to use it

The basic routine to create a state machine is the following:

1. Declare a FSM class

2. Declare all state classes

3. Instantiate FSM class

Declaring a FSM class

FSM class is the entry point of a FSM which is used to receive events (see below) and do the transitions between states. Each FSM must declare it’s own class which is a subclass of StateMachine. The simplest way is to just declare an empty class which inherits the class StateMachine:

from pyeds import fsm

class MyFsm(fsm.StateMachine):
    pass

Declaring a state class

Each state is represented by different class. Every method in that class may handle one particular event. To declare the state, a class must be decorated with DeclareState decorator which require state machine as an argument. This decorator binds the state class to the specific FSM class. Also, the new state class must be a subclass of State class:

@fsm.DeclareState(MyFsm)
class MyState(fsm.State):
    pass

Declare a new class per state.

Instantiating the FSM

To instantiate the FSM class do the following:

my_fsm = MyFsm()

After object initialization the FSM is put into running state.

Blinky example

The following is an example of FSM which is called Blinky. The FSM will print ‘on’ text and ‘off’ text on console with 0.5 seconds of delay between the messages.

The Blinky FSM has 2 states:

• State On

• State Off

o----+
     |
 On  v                Off
+----+----+  blink   +---------+
|         +--------->+         |
|         |          |         |
|         +<---------+         |
+---------+  blink   +---------+

The event blink is used to trigger transitions between the states.

from pyeds import fsm


# The first step is to declare a class which represent custom FSM.

class BlinkyFsm(fsm.StateMachine):
    pass


# The second step is to start writing the states of new state machine:

@fsm.DeclareState(BlinkyFsm)
class Initialization(fsm.State):
    def on_init(self):
        fsm.Every(0.5, fsm.Event('blink')
        return StateOn


@fsm.DeclareState(BlinkFsm)
class StateOn(fsm.State):
    def on_entry(self):
        print('on')

    def on_blink(self, event):
        return StateOff


@fsm.DeclareState(BlinkFsm)
class StateOff(fsm.State):
    def on_entry(self):
        print('off')

    def on_blink(self, event):
        return StateOn


# The final step is to instantiate the FSM class defined in the first step.

blinky_fsm = BlinkyFsm()

After creation the FSM is automatically put into a running state.

Event

An event is a notable occurrence at a particular point in time. Events can, but do not necessarily, cause state transitions from one state to another in state machines

An event can have associated parameters, allowing the event to convey not only the occurrence but also quantitative information about the occurrence.

An event in PyEDS is instanced using class Event.

The associated parameters with an event are:

• name of the event

• producer of event

Generate an event

To generate a new event just instantiate Event class with event name as parameter:

new_event = fsm.Event('my_special_event')

Alternative way is to first declare a new event class and instantiate this derive class:

class MySpecialEvent(fsm.Event):
    pass

new_event = MySpecialEvent() # This event is implicitly called 'my_special_event'

In this case base Event class will implicitly take the name of the class as own name. This can be overriden by calling the super constructor:

# This event has the exact same name as the above one
class MySecondEvent(fsm.Event):
    def __init__(self):
        super().__init__('my_special_event')

# Another way of creating event with same name as above events
class MyThirdEvent(fsm.Event):
    name = 'my_special_event'

Event class attributes and methods

Attributes:

• self.name - this is a string containing event name

• self.producer - specifies which state machine has generated this event.

Methods:

• release(self) - this method is called by state machine when it has finished the processing of the event

• execute(self, handler) - this method is called by state machine and it is used to modify how an event handler is called.

Rules about event naming

When an event is created and sent to a state machine it’s name is used to decide which method in current state instance should be invoked. The state machine takes the name of the event, it prepends text on_ to the name string and then it looks up to event handler method.

Example: If an event named toggle is created and sent to a state machine, the target state machine will lookup for a method named on_toggle in the current state instance.

Since the event name directly impacts which state instance method will be called the name of events must follow the Python identifier naming rules.

A Python identifier starts with a letter A to Z or a to z or an underscore (_) followed by zero or more letters, underscores and digits (0 to 9). Python does not allow punctuation characters such as @, $, and % within identifiers.

ok_event = fsm.Event('some_event_with_long_name')
bad_event = fsm.Event('you cannot use spaces, @, $ and % here')

State

A state is a description of the status of a system that is waiting to execute a transition.

State attributes and methods

Attributes:

• self.name - this is a string containing state name

• self.producer - specifies which state machine has this state

• self.sm - the same as self.producer but shorter

• self.logger - this is the logger which is used by state machine

• self.rm - this is ResourceManager for this state

• super_state - this is a class attribute that specifies super state class

Methods:

• release(self) - this method is called by state machine just before state machine termination

• on_entry(self) - this method is called by state machine when it has entered the state

• on_exit(self) - this method is called by state machine when it has exited the state

• on_init(self) - this method is called by state machine when it has entered the state and now needs to initialize the state

• on_unhandled_event - this method is called by state machine when no event handlers where found for this state

State machine

State machine attributes and methods

Attributes:

• self.name - this is a string containing Sstate machine name

• self.logger - this is the logger which is used by state machine

• self.rm - this is ResourceManager for this state machine

• self.state - current state of this machine

Methods:

• run(self) - this is state machine dispatch method

• put(self, event) - this method puts an event to state machine wait queue

• terminate(self) - pend termination of the state machine. After exiting this method the state machine may still run. Use self.wait to wait for FSM termination

• wait(self) - wait for FSM to terminate

• instance_of - get the instance of a state class

• on_terminate - gets called by state machine just before termination

• on_exception - gets called when unhandled exception has occured

State transition

Switching from one state to another is called state transition. A transition is a set of actions to be executed when a condition is fulfilled or when an event is received.

Transitions are started by returning target state class in an event handler.

def on_some_event(self, event):
    do_some_stuff()
    return SomeOtherState # Note: return a class object, not instance object

Hierarchical Finite State Machines (HFSM)

Please, refer to Wikipedia article for further explanation:

• https://en.wikipedia.org/wiki/UML_state_machine#Hierarchically_nested_states

Source

Source is available at github:

• https://github.com/nradulovic/pyeds