borg.py

class ExampleClass:
    classVar = "classVar"
    def __init__(self):
        self.instanceVar = "instanceVar"

class BorgClass:
    __shared_state = {}
    classVar = "classVar"
    def __init__(self):
        self.__dict__ = self.__shared_state        
        self.instanceVar = "instanceVar"

print "Assume this class:\n"
print "class ExampleClass:"
print "   classVar = 'classVar'"
print "   def __init__(self):"
print "      self.instanceVar = 'instanceVar'\n"

print "(1) classVars can be printed before an instance of the class exists:\n"
print ">>> ExampleClass.classVar"
print ExampleClass.classVar, "\n"

print "(2) instanceVars can only be printed once an instance exists:\n"
print ">>> ExampleClass.instanceVar"
print "Traceback (most recent call last):"
print "  File '<stdin>', line 1, in <module>"
print "AttributeError: class ExampleClass has no attribute 'instanceVar'\n"

print "(3) Setting a class variable from an instance of the class doesn't quite do what we'd expect:\n" 
print ">>> a = ExampleClass()"
a = ExampleClass()
print ">>> b = ExampleClass()"
b = ExampleClass()
print ">>> a.classVar = 'fizzbuzz'"
a.classVar = "fizzbuzz"
print ">>> print a.classVar"
print a.classVar
print ">>> print b.classVar"
print b.classVar, "\n"

print "(3b) We can achieve a unified class variable by adding the __class__ reference:\n"
print ">>> c = ExampleClass()"
c = ExampleClass()
print ">>> d = ExampleClass()"
d = ExampleClass()
print ">>> c.__class__.classVar = 'fizzbuzz'"
c.__class__.classVar = "fizzbuzz"
print ">>> print c.classVar"
print c.classVar
print ">>> print d.classVar"
print d.classVar, "\n"

#*******************************************************88

class BorgClass:
    __shared_state = {}     # empty dict 
    classVar = "classVar"   # class variable

    def __init__(self):
        self.__dict__ = self.__shared_state        
        self.instanceVar = "instanceVar"

print "Now assume this 'Borg' class:\n"
print "class BorgClass:"
print "   __shared_state = {}   # empty dict"
print "   classVar = 'classVar' # class variable\n"
print "   def __init__(self):"
print "      self.__dict__ = self.__shared_state # bind individual dicts to class variable dict"
print "      self.instanceVar = 'instanceVar'    # instance var\n"

print "(4) the Borg nature of this class means that class vars will change 'as one':\n" 
print ">>> e = BorgClass()"
e = BorgClass()
print ">>> f = BorgClass()"
f = BorgClass()
print ">>> e.classVar = 'fizzbuzz'"
e.classVar = "fizzbuzz"
print ">>> print e.classVar"
print e.classVar
print ">>> print f.classVar"
print f.classVar, "\n"

print "(5) even instance variables move 'as one' in a Borg class:\n"
print ">>> print e.instanceVar"
print e.instanceVar
print ">>> print f.instanceVar"
print f.instanceVar
print ">>> e.instanceVar = 'modified text'"
e.instanceVar = "modified text"
print ">>> print e.instanceVar"
print e.instanceVar
print ">>> print f.instanceVar"
print f.instanceVar