Incrementing class variables dynamically in Python

Question

I have a simple class, and every time I create an instance of that class, I want the class variable to increment how is should i do that with this code:

class Person:

    person_count = 0

    def __init__(self, username):
        self.username = username

ashley = Person("Ash")
daphne = Person("Daph")

Person.person_count #I want this to be 2

Answer 1

Simply increment the class variable in __init__:

class Person(object):

    person_count = 0

    def __init__(self, username):
        self.username = username
        Person.person_count += 1  # here

ashley = Person("Ash")
daphne = Person("Daph")

print(Person.person_count)
# 2

And don't forget to subclass from object if you're on Python 2.

See What is the purpose of subclassing the class "object" in Python?

Answer 2

You will have to increment the class's variable within the __init__ as:

class Person:
    person_count = 0
    def __init__(self, username):
        self.username = username
        self.__class__.person_count += 1
        # OR, 
        # Person.person_count += 1

Example:

>>> ashley = Person("Ash")
>>> ashley.person_count
1
>>> daphne = Person("Daph")
>>> daphne.person_count
2

You may also extract the count directly using class as:

>>> Person.person_count
2