Python Event Driven System

Table of contents

• Introduction
• Installation
  • Using PIP
  • Installing from source
  • Code documentation
• How to use it
  • Declaring a FSM class
  • Declaring a state class
  • Instantiating the FSM
• Blinky example
• Event
  • Generate an event
  • Rules about event naming
  • Events with parameters
• Timers
• State
  • State members
  • State hierarchy
  • State owner
• State machine
• State transition
• Hierarchical Finite State Machines (HFSM)
• Source
• Other links

Introduction

This package provides a means to efficiently write finite state machines (FSM) by hand. The focus was to make the API as simple as possible since no GUI tools are included to define a FSM. This package allows you to create state machines with/without state hieararchy.

Installation

Using PIP

PyEDS can be installed using the standard Python tool pip with

pip install pyeds

Installing from source

The easiest way to install PyEDS from source is to use setup.py script which uses setuptools. For complete documentation about this script please refer to setuptools manual.

To install from source issue the following command:

python setup.py install

Code documentation

The documentation is available online at ReadTheDocs.

Code documentation is bundled together with the source. The documentation scripts use Sphinx to generate documents. To generate the documentation from code please refer to docs/README.rst.

The documentation can be accessed via Python interpreter, too.

>>> import pyeds
>>> help(pyeds.fsm)

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, 'blink')
        return StateOn


@fsm.DeclareState(BlinkyFsm)
class StateOn(fsm.State):
    def on_entry(self):
        print('on')
        # on_entry must not return state class as other event handlers

    def on_blink(self, event):
        return StateOff


@fsm.DeclareState(BlinkyFsm)
class StateOff(fsm.State):
    def on_entry(self):
        print('off')
        # on_entry must not return state class as other event handlers

    def on_blink(self, event):
        return StateOn


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

blinky_fsm = BlinkyFsm()
blinky_fsm.wait(4)  # Wait it for 4 seconds and terminate it

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 is the only means of communication between state machines. Each event carries name. Based on the event name a handler will be called from current state class which has the same name.

An event in PyEDS is instanced using class Event.

The associated parameters with an event are:

• Name of the event: this is a string containing event name.
• Owner of event: specifies which state machine has generated this 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 derived 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 overridden by calling the super constructor:

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

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; please refer to https://docs.python.org/3.3/reference/lexical_analysis.html#identifiers for more details.

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

Events with parameters

Each event may carry additional parameters describing the event. For example, you can create event classes that suit your needs:

class AxisButtonPress(fsm.Event):
    def __init__(self, direction):
        super().__init__()
        self.direction = direction

then in some FSM state:

@fsm.DeclareState(MyFsm)
class Initialization(fsm.State):
    def on_axis_button_press(self, event):
        print(event.direction)

Timers

Timers are used to generate time events:

• After: Means an event will be generated after elapsed time.
• Every: Means an event will be generated every period of time.

To generate the events use After and Every objects:

@fsm.DeclareState(BlinkyFsm)
class Initialization(fsm.State):
    def on_init(self):
        self.blinking = fsm.Every(1.0, 'blink')
        return StateOn

This line will generate an event named blink every 1.0 seconds. To stop the timer use:

@fsm.DeclareState(BlinkyFsm)
    class StateOn(fsm.State):
        def on_entry(self):
            print('on')
            self.blinking.cancel()
            # on_entry must not return state class as other event handlers

Second approach to cancel a running timer is by using event timer attribute. When a timer generates an event it will automatically create event attribute called timer. With this attribute you can also access the originating timer through event. To stop the timer through an event see the example below:

@fsm.DeclareState(BlinkyFsm)
class StateOn(fsm.State):
    def on_blink(self, event):
        event.timer.cancel() # Stop the originating timer
        return StateOff

State

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

State contains function which correspond to events which are to be processed by state. When a state is able to process an event it is said that it is sensitive to that event. In the following example state State_A is sensitive to two events:

• event_1 - Which is handled by on_event_1 function. After the event is processed the state machine will transition to State_B.
• event_2 - Which is handled by on_event_2 function. After the event is processed the state machine will remain in State_A state (not taking the transition).

@fsm.DeclareState(MyFsm)
class State_A(fsm.State):
    def on_event_1(self, event):
        # Process event event_1
        return State_B

    def on_event_2(self, event):
        # Process event event_2

State members

Each state has the following members:

• super_state - Specifies the state hierarchy
• sm - The state machine who is owner of this state.
• logger - Logger of the state machine

State hierarchy

Finite-state machine states can have a hierarchy. When you want to declare that a state is substate of a state use super_state attribute of State class:

@fsm.DeclareState(MyStateMachine)
class SuperState(fsm.State):
    pass

@fsm.DeclareState(MyStateMachine)
class SubState(fsm.State):
    super_state = SuperState

By default super_state is set to None which means that the state has no super state, in other words, it is a top level state.

State owner

Each state instances is owned by an instance of state machine. The sm property allows acccess to the instance of state machine from state instance.

For example, let's say you have FSM with the following definition:

class MyFsm(fsm.StateMachine):
    A_VARIABLE = 13

You can access A_VARIABLE from any state of the state machine with:

@fsm.DeclareState(MyFsm)
class MyState(fsm.State):
    def on_entry(self):
        print(self.sm.A_VARIABLE)
        # on_entry must not return state class as other event handlers

State machine

A finite-state machine (FSM) is a mathematical model of computation. It is an abstract machine that can be in exactly one of a finite number of states at any given time. The FSM can change from one state to another in response to some external events; the change from one state to another is called a state transition. An FSM is defined by a list of its states, its initial state, and the conditions for each transition.

A state machine is automatically started as soon as they are created. If you don't want this set the class attribute should_autostart to False:

class BlinkyFsm(fsm.StateMachine):
    should_autostart = False

# Create states here

blinky_fsm = BlinkyFsm()
blinky_fsm.do_start()
blinky_fsm.wait(4)  # Wait it for 4 seconds and terminate it

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

If event function returns None then the state machine will not start the transition to any state (it will stay in the current one).

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

Other links

The following is a list of links to tools used by the project:

• Sphinx (used to build documentation): http://www.sphinx-doc.org/en/stable/
• setuptools (used for installing from source): https://setuptools.readthedocs.io/en/latest/