Class variables in Python

A class variable is defined in the class body, outside any method. There is one copy of it, shared by every object. An instance variable is set on self, and each object has its own.

The program

class Player:
    count = 0  # class variable: one value for the whole class

    def __init__(self, name):
        self.name = name  # instance variable: one per player
        Player.count += 1

    @classmethod
    def how_many(cls):
        return cls.count


ada = Player("Ada")
alan = Player("Alan")
grace = Player("Grace")

print("Players created:", Player.how_many())
print("Read through an object:", ada.count)
print("Names:", ada.name, alan.name, grace.name)

Players created: 3
Read through an object: 3
Names: Ada Alan Grace

How it works

• Player.count += 1 changes the class variable, so every new player adds 1 to the same counter.
• ada.count also works for reading. Python looks for count on the object first, does not find it, and then looks on the class.
• @classmethod makes a method that receives the class (cls) instead of an object. You can call it on the class, as in Player.how_many(), without creating a player.
• Write Player.count += 1, not self.count += 1. The second one creates a new instance variable on that one object, and the shared counter stays at 0.

A shared list by mistake

A class variable that holds a list is shared too. This is a common bug:

class Team:
    members = []  # shared by every team

    def add(self, name):
        self.members.append(name)


red = Team()
blue = Team()
red.add("Linus")
print("Blue team:", blue.members)


class FixedTeam:
    def __init__(self):
        self.members = []  # a new list for each team

    def add(self, name):
        self.members.append(name)


red = FixedTeam()
blue = FixedTeam()
red.add("Linus")
print("Blue team:", blue.members)

Blue team: ['Linus']
Blue team: []

self.members.append(...) does not create anything new on the object. It finds the one list on the class and changes it. Create the list in __init__ so that each object gets its own.