Classes and Objects

So far you have used types that Python provides: str, list, dict. A class lets you create your own type. An object is one value made from that class, just as "hi" is one object of the str class.

Python

Open in playground

class Dog:
    def __init__(self, name, age):
        self.name = name
        self.age = age

    def bark(self):
        return f"{self.name} says woof!"

rex = Dog("Rex", 3)
tommy = Dog("Tommy", 5)
print(rex.bark())
print(tommy.name, tommy.age)

Output

Rex says woof!
Tommy 5

__init__ is the constructor. It runs automatically when you create an object and sets up its data.
self refers to the object the method is working on.
Values stored on self, such as self.name, are called attributes.
Functions defined inside a class are called methods.

Changing an object's state

Methods can read and update the object's attributes.

Python

Open in playground

class BankAccount:
    def __init__(self, owner, balance=0):
        self.owner = owner
        self.balance = balance

    def deposit(self, amount):
        self.balance += amount

    def withdraw(self, amount):
        if amount > self.balance:
            print("Not enough money!")
            return
        self.balance -= amount

acc = BankAccount("Meera", 1000)
acc.deposit(500)
acc.withdraw(2000)
acc.withdraw(300)
print(acc.owner, acc.balance)

Output

Not enough money!
Meera 1200

A readable printout with str

By default, printing an object shows something like <__main__.Point object at 0x...>. Define __str__ to control what print() shows.

Python

Open in playground

class Point:
    def __init__(self, x, y):
        self.x = x
        self.y = y

    def distance_from_origin(self):
        return (self.x ** 2 + self.y ** 2) ** 0.5

    def __str__(self):
        return f"Point({self.x}, {self.y})"

p = Point(3, 4)
print(p)
print(p.distance_from_origin())

Output

Point(3, 4)
5.0

Changing an object's state

A readable printout with __str__

A readable printout with str