Classes and objects

Lesson 27 of 62, about 3 min

A class is a blueprint for creating objects. An object bundles data (attributes) with the functions that operate on that data (methods). Almost everything in Python is an object, including numbers, strings and functions.

Defining a class

python

class Dog:
    pass

d = Dog()
print(type(d))

Class names use PascalCase. Calling the class like a function creates an instance.

The init method

__init__ runs automatically when an instance is created. It is where you set up the object's attributes.

python

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

buddy = Dog("Buddy", 3)
print(buddy.name, buddy.age)

self is the instance being created. Python passes it automatically as the first argument to every method; you never pass it yourself.

Methods

Functions defined inside a class. They always take self first so they can reach the instance's attributes.

python

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

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

    def rename(self, new_name):
        self.name = new_name

d = Dog("Rex")
print(d.speak())
d.rename("Max")
print(d.speak())

Attributes can be changed

python

class Account:
    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:
            return "Insufficient funds"
        self.balance -= amount
        return self.balance

acc = Account("Meena", 500)
acc.deposit(250)
print(acc.withdraw(100))
print(acc.withdraw(5000))
print(acc.balance)

Class attributes

Defined directly in the class body, shared by every instance.

python

class Dog:
    species = "Canis familiaris"

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

a = Dog("A")
b = Dog("B")
print(a.species, b.species)
Dog.species = "dog"
print(a.species)

Use them for constants and defaults. Be careful with mutable class attributes like lists; they are shared too.

str for printing

By default printing an object shows something unhelpful. Define __str__ to control it.

python

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

    def __str__(self):
        return f"Dog named {self.name}"

print(Dog("Bruno"))

More special methods are covered in the special methods lesson.

Objects in collections

python

class Student:
    def __init__(self, name, marks):
        self.name = name
        self.marks = marks

roster = [Student("Ali", 82), Student("Bela", 91), Student("Chen", 77)]
best = max(roster, key=lambda s: s.marks)
print(best.name)
for s in sorted(roster, key=lambda s: s.name):
    print(s.name, s.marks)

Deleting

python

class Thing:
    pass

t = Thing()
t.colour = "red"     # attributes can be added later
del t.colour
del t

Practice

Write a Rectangle class with width and height attributes and methods area() and perimeter().
Add a __str__ method so print(Rectangle(3, 4)) shows Rectangle 3x4.
Write a Counter class with an increment() method and a class attribute that counts how many Counter objects have been created.