Design Patterns State in Python

Radhika Sharma

a year ago

Python provides a module for the state machines, which are implemented using subclasses, which are derived from a specified state machine class. The methods are state independent and cause transition declared using decorators.
Implementation of the state pattern?
Let’s take a simple example to see the implantation of the state pattern.

class ComputerState(object):

   name = "state"
   allowed = []

   def switch(self, state):
      """ Switch to new state """
      if state.name in self.allowed:
         print('Current:',self,' => switched to new state',state.name)
         self.__class__ = state
      else:
         print ('Current:',self,' => switching to',state.name,'not possible.')

   def __str__(self):
      return self.name

class Off(ComputerState):
   name = "off"
   allowed = ['on']

class On(ComputerState):
   """ State of being powered on and working """
   name = "on"
   allowed = ['off','suspend','hibernate']

class Suspend(ComputerState):
   """ State of being in suspended mode after switched on """
   name = "suspend"
   allowed = ['on']

class Hibernate(ComputerState):
   """ State of being in hibernation after powered on """
   name = "hibernate"
   allowed = ['on']

class Computer(object):
   """ A class representing a computer """
  
   def __init__(self, model='HP'):
      self.model = model
      # State of the computer - default is off.
      self.state = Off()
  
   def change(self, state):
      """ Change state """
      self.state.switch(state)

if __name__ == "__main__":
   comp = Computer()
   comp.change(On)
   comp.change(Off)
   comp.change(On)
   comp.change(Suspend)
   comp.change(Hibernate)
   comp.change(On)
   comp.change(Off)
Output
The output of the above program-
Output | Insideaiml
Output | Insideaiml

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