#!/usr/bin/env python

# Written by: DGC

# python imports

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

class MonoState(object):

__data = 5

@property

def data(self):

return self.__class__.__data

@data.setter

def data(self, value):

self.__class__.__data = value

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

class MonoState2(object):

pass

def add_monostate_property(cls, name, initial_value):

"""

Adds a property "name" to the class "cls" (should pass in a class object

not a class instance) with the value "initial_value".

This property is a monostate property so all instances of the class will

have the same value property. You can think of it being a singleton

property, the class instances will be different but the property will

always be the same.

This will add a variable __"name" to the class which is the internal

storage for the property.

Example usage:

class MonoState(object):

pass

add_monostate_property(MonoState, "data", 5)

m = MonoState()

# returns 5

m.data

"""

internal_name = "__" + name

def getter(self):

return getattr(self.__class__, internal_name)

def setter(self, value):

setattr(self.__class__, internal_name, value)

def deleter(self):

delattr(self.__class__, internal_name)

prop = property(getter, setter, deleter, "monostate variable: " + name)

# set the internal attribute

setattr(cls, internal_name, initial_value)

# set the accesser property

setattr(cls, name, prop)

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

if (__name__ == "__main__"):

print("Using a class:")

class_1 = MonoState()

print("First data: " + str(class_1.data))

class_1.data = 4

class_2 = MonoState()

print("Second data: " + str(class_2.data))

print("First instance: " + str(class_1))

print("Second instance: " + str(class_2))

print("These are not singletons, so these are different instances")

print("")

print("")

print("Dynamically adding the property:")

add_monostate_property(MonoState2, "data", 5)

dynamic_1 = MonoState2()

print("First data: " + str(dynamic_1.data))

dynamic_1.data = 4

dynamic_2 = MonoState2()

print("Second data: " + str(dynamic_2.data))

print("First instance: " + str(dynamic_1))

print("Second instance: " + str(dynamic_2))

print("These are not singletons, so these are different instances")