class Test:
counter = 0
def __init__(self):
type(self).counter += 1
def __del__(self):
type(self).counter -= 1
o1 = Test()
print("instance: " + str(Test.counter))
o2 = Test()
print("instance: " + str(Test.counter))
o3 = Test()
print("instance: " + str(Test.counter))
del o1
print("instance: " + str(Test.counter))
del o2
print("instance: " + str(Test.counter))
'''
run:
instance: 1
instance: 2
instance: 3
instance: 2
instance: 1
'''