# Singleton design pattern example.

# For more information on singleton:
# http://en.wikipedia.org/wiki/Singleton_pattern
# http://www.vincehuston.org/dp/singleton.html

class Singleton(type):
    # This is a Gary Robinson implementation:
    # http://www.garyrobinson.net/2004/03/python_singleto.html
    def __init__(cls,name,bases,dic):
        super(Singleton,cls).__init__(name,bases,dic)
        cls.instance=None
        
    def __call__(cls,*args,**kw):
        if cls.instance is None:
            cls.instance=super(Singleton,cls).__call__(*args,**kw)
        return cls.instance

class Bone(object):
    # Only one project will be initialized per time. Therefore, we use
    # Singleton design pattern for implementing it
    __metaclass__= Singleton
    
    def __init__(self):
        self.size = 100
        
    def RemovePart(self, part_size):
        self.size -= part_size # self.size = self.size - part_size

class Dog():
    def __init__(self, name):
        self.name = name
        self.bone = Bone()
        
    def EatBonePart(self, part_size):
        self.bone.RemovePart(part_size) 

print "Initial state:"
d1 = Dog("Nina")
d2 = Dog("Tang")
print "Bone size of %s: %d"%(d1.name, d1.bone.size)
print "Bone size of %s: %d\n"%(d2.name, d2.bone.size)

print "Only Nina eats:"
d1.EatBonePart(5)
print "Bone size of %s: %d"%(d1.name, d1.bone.size)
print "Bone size of %s: %d\n"%(d2.name, d2.bone.size)

print "Tang eats after Nina:"
d2.EatBonePart(20)
print "Bone size of %s: %d"%(d1.name, d1.bone.size)
print "Bone size of %s: %d"%(d2.name, d2.bone.size)