instance_counting.py

from collections import defaultdict


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

    def __init__(self):
        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__]


class Puppy(Dog):
    pass


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

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

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

thinkpad[~/Projects/ark] python2 test.py 
Dog: 1
Pup: 1
Dog: 1
Pup: 0