Introduction

A finite state machine (FSM) is a mathematical model of computation with states, transitions, inputs and outputs. This machine is always in a one state at the time and can move to other states using transitions. A transition changes the state of the machine to another state.

A large number of problems can be modeled using finite state machines. Simple examples of state machines used in modern life are vending machines, elevators and traffic lights.  Advanced usage are artificial intelligence, language parsing and communication protocol design.

Related course
Practice Python with interactive exercises

Finite State Machine Example

First install the Fysom module:

sudo pip install fysom

We can define a Finite State Machine (FSM) with two states: sleeping and awake.  To move between the states we will define the transitions wakeup() and sleep().

Finite state machine. States: awake, sleeping. Transitions: sleep, wake up

Example:

from fysom import *
fsm = Fysom({'initial': 'awake',
             'final': 'red',
             'events': [
                 {'name': 'wakeup', 'src': 'sleeping', 'dst': 'awake'},
                 {'name': 'sleep',  'src': 'awake',   'dst': 'sleeping'}]})
print(fsm.current)   # awake
fsm.sleep()
print(fsm.current)   # sleeping
fsm.wakeup()
print(fsm.current)   # awake

Result:

awake
sleeping
awake

Finite State Machines

There are several implementations of Finite State Machines in Python:

• Fysom
• Python-bits fsm
• Fsme