Classes and OOP I

Class Attributes

So far every attribute lived on an individual instance. Sometimes you want data that is shared across all instances of a class. That is what class attributes are for.

So far every attribute you created lived on a single instance. But sometimes a piece of data belongs to the class itself and should be shared by all instances.

A class attribute is defined directly in the class body, outside any method:

class Dog:
	species = "Canis familiaris"
 
	def __init__(self, name):
		self.name = name

Python

Every Dog instance can read the shared species attribute:

class Dog:
	species = "Canis familiaris"
 
	def __init__(self, name):
		self.name = name
 
rex = Dog("Rex")
buddy = Dog("Buddy")
print(rex.species)
print(buddy.species)

Python

Output

What will be the output?

class Cat:
	legs = 4
 
	def __init__(self, name):
		self.name = name
 
c = Cat("Luna")
print(c.legs)

Python

You can also access a class attribute through the class name directly, without any instance.

Both ways work. Through the class or through an instance:

class Cat:
	legs = 4
 
print(Cat.legs)
c = Cat("Luna")
print(c.legs)

Python

Output

A class attribute can be updated through the class name. The change is visible to all instances.

Watch what happens when we change the class attribute:

class Config:
	mode = "dark"
 
a = Config()
b = Config()
Config.mode = "light"
print(a.mode)
print(b.mode)

Python

Output

What will be the output?

class App:
	version = 1
 
a = App()
App.version = 2
b = App()
print(a.version, b.version)

Python

But setting an attribute through an instance creates a new instance attribute that shadows the class one. The class attribute stays unchanged.

Only a gets a new instance attribute. b still reads the class attribute:

class Item:
	color = "red"
 
a = Item()
b = Item()
a.color = "blue"
print(a.color)
print(b.color)
print(Item.color)

Python

Output

What will be the output?

class Box:
	size = "small"
 
x = Box()
y = Box()
x.size = "large"
print(y.size)

Python

A common use case is counting how many instances have been created. The counter lives on the class so every new instance can update it.

Each __init__ call increments the shared counter:

class Player:
	count = 0
 
	def __init__(self, name):
		self.name = name
		Player.count += 1
 
Player("A")
Player("B")
print(Player.count)

Python

Output

What will be the output?

class Ticket:
	sold = 0
 
	def __init__(self):
		Ticket.sold += 1
 
Ticket()
Ticket()
Ticket()
print(Ticket.sold)

Python

What will be the output?

class Room:
	max_cap = 30
 
	def __init__(self, guests):
		self.guests = guests
 
	def is_full(self):
		return self.guests >= Room.max_cap
 
r = Room(30)
print(r.is_full())

Python

What will be the output?

class Light:
	brightness = 50
 
a = Light()
b = Light()
a.brightness = 100
Light.brightness = 75
print(a.brightness)
print(b.brightness)

Python