Counting instances of a class?

Here is a way to count instances without descendant classes sharing the same id/count. A metaclass is used to create a separate id counter for each class.

Uses Python 3 syntax for Metaclasses.

import itertools

class InstanceCounterMeta(type):
    """ Metaclass to make instance counter not share count with descendants
    """
    def __init__(cls, name, bases, attrs):
        super().__init__(name, bases, attrs)
        cls._ids = itertools.count(1)

class InstanceCounter(object, metaclass=InstanceCounterMeta):
    """ Mixin to add automatic ID generation
    """
    def __init__(self):
        self.id = next(self.__class__._ids)

You could consider using a class attribute to provide a counter. Each instance needs only to ask for the next value. They each get something unique. Eg:

from itertools import count

class Obj(object):
  _ids = count(0)

  def __init__(self):
    self.id = next(self._ids)

This should do the job:

class Obj:
    _counter = 0
    def __init__(self):
        Obj._counter += 1
        self.id = Obj._counter