Classes and instances

A template for objects

A class describes a kind of thing; an instance is one of them.

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

rex = Dog("Rex", "collie")
rex.name       # "Rex"

__init__ runs automatically when you create an instance. It is not a constructor in the C++ sense — the object already exists — it is where you set the object up.

self is the instance being worked on. It is the first parameter of every method, and Python passes it for you: you write Dog("Rex", "collie") with two arguments, and __init__ receives three.

Attributes assigned to self belong to that one instance. Two dogs have two names.

Your turn: write a Book class taking title, author and pages, storing each on the instance.

Methods

A method is a function that lives on a class and receives the instance as self:

class Counter:
    def __init__(self):
        self.value = 0

    def increment(self, by=1):
        self.value += by
        return self.value

Inside a method, self.value reads or writes that instance's attribute. Forgetting self. is the usual early mistake — a bare value = 0 creates a local variable that disappears the moment the method returns.

Your turn: give Book a reading_time(pages_per_hour=40) method returning how many hours it would take, rounded to one decimal place.