Classes and Objects

Introduction

A dictionary can hold the data for a dog: name, age. But what the dog can do (bark, have a birthday) lives somewhere else, in separate functions. A class keeps the data and the behaviour together in one package, called an object. This style is named object-oriented programming.

The simple explanation

A class is a blueprint. An object is a thing built from that blueprint.

• class Dog: defines the blueprint. Class names use CapitalWords.
• __init__ is the constructor. It runs when you create an object with Dog("Rex", 3) and sets up its data.
• self is the object itself. Every method gets it as its first parameter, and self.name is a value stored on that object.
• Each object has its own data. rex and bella are independent.

Printing objects nicely

By default, printing an object shows an unhelpful label. Special methods (with double underscores, called dunder methods) let you customise behaviour.

__str__ is for people reading output, and __repr__ is for developers and debugging.

Class attributes

A variable defined on the class itself is shared by all objects.

Inheritance

A class can inherit from another to reuse and specialise it. The child gets everything from the parent, and can add or replace methods.

super() calls the parent's version. Treating different classes through the same method name, as in the loop, is called polymorphism.

Properties: controlled attributes

Use @property to make a method look like an attribute, perhaps with checks.

By convention, a name starting with an underscore (_celsius) means "internal, please do not touch". Python does not enforce it. The last line raises the error from the setter.

Composition: has-a instead of is-a

Inheritance means "is a" (a Dog is an Animal). Often "has a" is clearer: a Library has books.

Try it yourself

Create a class BankAccount with an owner and a balance (starting at 0), a deposit(amount) method and a withdraw(amount) method that raises ValueError when there is not enough money. Add a __str__ that shows the owner and balance.

Summary

• A class is a blueprint, and __init__ sets up each new object. self is the object.
• Special methods like __str__, __eq__ and __add__ customise how objects behave.
• A child class inherits from its parent. super() reaches the parent, and each class can override methods.
• @property gives controlled, attribute-style access. Prefer composition when the relationship is "has a".