#!/usr/bin/env python

# Written by: DGC

import abc

#==============================================================================

class Singleton(object):

""" A generic base class to derive any singleton class from. """

__metaclass__ = abc.ABCMeta

__instance = None

def __new__(new_singleton, *arguments, **keyword_arguments):

"""Override the __new__ method so that it is a singleton."""

if new_singleton.__instance is None:

new_singleton.__instance = object.__new__(new_singleton)

new_singleton.__instance.init(*arguments, **keyword_arguments)

return new_singleton.__instance

@abc.abstractmethod

def init(self, *arguments, **keyword_arguments):

"""

as __init__ will be called on every new instance of a base class of

Singleton we need a function for initialisation. This will only be

called once regardless of how many instances of Singleton are made.

"""

raise

#==============================================================================

class GlobalState(Singleton):

def init(self):

self.value = 0

print("init() called once")

print("")

def __init__(self):

print("__init__() always called")

print("")

class DerivedGlobalState(GlobalState):

def __init__(self):

print("derived made")

super(DerivedGlobalState, self).__init__()

def thing(self):

print(self.value)

#==============================================================================

if (__name__ == "__main__"):

d = DerivedGlobalState()

print(type(d))

d.thing()

d.value = -20

e = DerivedGlobalState()

e.thing()

f = DerivedGlobalState()

f.thing()

a = GlobalState()

# value is default, 0

print("Expecting 0, value = %i" %(a.value))

print("")

# set the value to 5

a.value = 5

# make a new object, the value will still be 5

b = GlobalState()

print("Expecting 5, value = %i" %(b.value))

print("")

print("Is a == b? " + str(a == b))