count_instances.py

from collections import defaultdict


class Dog(object):
    __instance_count = defaultdict(int)

    def __init__(self, colour='white', breed='mutt'):
        self.colour = colour
        self.breed = breed

        self.__instance_count[self.__class__.__name__] += 1

    def __del__(self):
        self.__instance_count[self.__class__.__name__] -= 1

    @classmethod
    def count(cls):
        return cls.__instance_count[cls.__name__]

    @classmethod
    def count_all(cls):
        return sum(cls.__instance_count.values())

    # Override these
    def speak(self):
        print 'wuf wuf'

    def __repr__(self):
        return 'this Dog is a %s %s' % (self.colour, self.beed)


class Puppy(Dog):
    def __init__(self, colour='white', breed='mutt', wormed=True):
        super(Puppy, self).__init__(colour, breed)
        self.wormed = wormed

    def speak(self):
        print 'ruf'

    def __repr__(self):
        worm_status = self.wormed and 'is dewormed' or 'might have worms'
        return 'this Puppy is a %s %s and %s' % (
            self.colour,
            self.breed,
            worm_status,
        )


if __name__ == '__main__':
    dog = Dog()
    pup = Puppy()

    print 'Dog:', dog.count()
    print 'Pup:', pup.count()
    print 'All:', Dog.count_all()

    del pup
    print 'Dog:', Dog.count()
    print 'Pup:', Puppy.count()
    print 'All:', Dog.count_all()

thinkpad[~/Projects/ark] python2 count_instances.py 
Dog: 1
Pup: 1
All: 2
Dog: 1
Pup: 0
All: 1