Counting instances per class using metaclass

countmeta.py

"""
A mechanism to keep track of no. of class instances created.
"""

from __future__ import print_function

class CountInstances(type):
	def __new__(meta, classname, supers, classdict):
		classdict['numInstances'] = 0
		return type.__new__(meta, classname, supers, classdict)
		
	def __call__(klass, *args, **kwargs):  
		klass.numInstances += 1
		return type.__call__(klass, *args, **kwargs)
		
	def printNumInstances(klass):
		print('No. of instances are:', klass.numInstances)
		
class A: 
	__metaclass__ = CountInstances
	def __init__(self):
		pass
	def ameth(self):
		print('A')

class B:
	__metaclass__ = CountInstances
	def __init__(self):
		pass
	def bmeth(self):
		print('B')

if __name__ == '__main__':
	a1, a2 = A(), A()
	b1, b2, b3 = B(), B(), B()

	a1.ameth()
	b1.bmeth()

	A.printNumInstances()
	B.printNumInstances()