PCC·09 · Chapter record

Classes

Chapter 9 — modeling real things as objects

Done

Five sessions on object-oriented Python: writing a class and making instances from it, default attributes and the three ways to modify them, inheritance, overriding a parent's method, and building classes out of other classes. Each one explains the concept, lists the key terms, then gives you exercises — try them yourself before revealing the code.

9.1

Creating and Using a Class

Concept

A class is a set of instructions for making an object — by convention its name is capitalized, like Dog.
__init__() is a special method Python runs automatically every time you create a new instance — it sets up that instance's starting attributes.
The first parameter of every method is self — a reference to the specific instance the method was called on. Python passes it automatically; you never supply it yourself.
self.name = name attaches a value to the instance as an attribute — from then on, every method (and you, from outside) can read it as instance.name.
Each instance is independent — creating a second Dog doesn't affect the first one's attributes at all.

Key Terms

class: a blueprint for creating objects
instance: one specific object made from a class
__init__(): runs automatically when a new instance is created
self: a reference to the instance a method was called on
attribute: a value attached to an instance via self.name = value

Exercise

Activity 1

Make a class Restaurant with restaurant_name and cuisine_type attributes, plus a describe_restaurant() method. Create an instance and call the method.

Activity 2

Create three different Restaurant instances and call describe_restaurant() for each — confirm they stay independent.

9.2

Default Values & Modifying Attributes

Concept

An attribute can get its value inside __init__() without being passed in — like self.odometer_reading = 0, a sensible starting default.
You can change an attribute's value directly through an instance: my_car.odometer_reading = 23.
Or write a method that updates it for you — which lets you add validation, like refusing to roll the odometer back.
Or write a method that increments it by some amount — self.odometer_reading += miles — rather than replacing the value outright.

Key Terms

default attribute: an attribute set inside __init__() without a parameter

Exercise

Activity 3

Add a number_served attribute (default 0) to your Restaurant class, plus set_number_served() and increment_number_served() methods.

Activity 4

Try to set a car's odometer to a lower number than it currently reads, and confirm the rollback-protection message appears.

9.3

Inheritance

Concept

A class can inherit from another: class ElectricCar(Car): makes ElectricCar a child of the parent class Car.
Call super().__init__(...) inside the child's __init__() to run the parent's setup — it initializes every attribute the parent defines, so you don't repeat that code.
The child inherits every method the parent has — a plain ElectricCar can already do everything a Car can, with zero extra code.
Then add whatever's specific to the child — like self.battery_size, which a plain Car doesn't have.

Key Terms

parent class / child class: the class being inherited from, and the class that inherits
super().__init__(): calls the parent class's __init__() from inside the child

Exercise

Activity 5

Write a class that inherits from one of your earlier classes (e.g. an IceCreamStand(Restaurant)) and adds one new attribute specific to the child.

9.4

Overriding Methods

Concept

Define a method in the child class with the same name as one in the parent, and Python uses the child's version instead — this is overriding.
It's the right move whenever the parent's behavior doesn't make sense for the child — an electric car filling a gas tank, for instance.
The parent's version of the method is still there — it's just ignored when you call it on an instance of the child.

Key Terms

overriding: defining a method in the child class to replace the parent's version

Exercise

Activity 6

Give your parent class a method that doesn't make sense for one of your child classes, then override it there with more appropriate behavior.

9.5

Instances as Attributes

Concept

When a class grows too many attributes and methods around one sub-topic, pull that sub-topic into its own class — like moving all the battery logic out of ElectricCar into a Battery class.
Then store an instance of that class as an attribute: self.battery = Battery().
To reach it from outside, chain through the attribute: my_tesla.battery.describe_battery() — instance, then its attribute, then a method on that.

Key Terms

composition: building a class out of instances of other classes

Exercise

Final challenge

Split one of your classes' more detailed attributes into a separate helper class (like an Engine, or a Menu), then store an instance of it as an attribute and reach it with dot-chaining.