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Python Event Driven System

Project description

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Python Event Driven System

Introduction

This package provides a means 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

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

Code documentation is bundled together with the source. The documentation scripts use Sphinx to generate documents.

To generate HTML documentation:

cd doc
make html

This will create HTML documents in doc/_build/html directory.

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(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 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')

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:

blinking = fsm.Every(1.0, 'blink')

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

blinking.cancel()

When a timer generates an event it will add new attribute to event called timer. With this attribute you can access the originating timer through event. This means that you can also stop the timer through an event:

def on_blink(self, event):
    event.timer.cancel() # Stop the originating timer

State

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

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.

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

Source

Source is available at github:

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