Counting instances of a class?

Question

I\'ve been cleaning up some code from a module I\'m extending and I can\'t seem to find a way to Pythonify this code:

global_next_id = 1

class Obj:
  def __         


        

Answer 1

This should do the job:

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

Answer 2

I found the following solution:

class Obj:
    counter = 0

    def __init__(self):
        type(self).counter += 1

    def __del__(self):
        type(self).counter -= 1

It's better to use type(self).counter instead of Obj.counter

Answer 3

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

my_obj = Obj()
print(Obj.global_next_id)

Answer 4

class InstanceCounter(object):
  # the instance counter
  counter = 0

  def __init__(self, val):
    self.val = all
    # incrementing every time an instance is created
    InstanceCounter.counter += 1

  def set_val(self, val):
    self.val = val

  def get_val(self, val):
    return self.val

  # accessing the instance counter should be done through a class method

  @classmethod
  def get_counter(cls):  
    return cls.counter

# See the instance counter as it increments as new instances are created
a=InstanceCounter(5)
print(a.get_counter())
b=InstanceCounter(7)
print(a.get_counter(), b.get_counter())
c=InstanceCounter(9)
print(a.get_counter(), b.get_counter(), c.get_counter())

Answer 5

Try something like this:

from itertools import count

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

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

Answer 6

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)