#  SingletonDemo.py  Copyright (c) Kari Laitinen

#  http://www.naturalprogramming.com

#  2011-12-12 File created.
#  2022-12-27  Converted to Python 3.

#  This program demonstrates how to implement 
#  a design pattern called the singleton pattern.

#  In software engineering, the singleton pattern is
#  a design pattern used to implement the mathematical
#  concept of a singleton, by restricting the
#  instantiation of a class to one object.
#  This is useful when exactly one object is needed
#  to coordinate actions across the system.
#  The concept is sometimes generalized to systems
#  that operate more efficiently when only one object
#  exists, or that restrict the instantiation to a certain
#  number of objects.

class SystemData :

   instance_to_return = None

   def __init__( self )  :

      self.some_data_member = 222
      self.another_member   = "Test Data"


   @staticmethod
   def get_instance() :
   
      if SystemData.instance_to_return == None :
      
         SystemData.instance_to_return = SystemData()
         
      return SystemData.instance_to_return


#  Testing

system_data  =  SystemData.get_instance()

print( system_data.some_data_member )
print( system_data.another_member )