Classes and Objects — PythonLessonLab

THE IDEA

Understand the concept

A class describes how objects of one kind are built. The __init__ method sets up each new object, and self refers to the particular instance being created or used.

An instance can store data in attributes such as self.title. Methods are functions attached to the class. Calling an instance method passes that instance as self automatically.

class defines a type
__init__ sets instance attributes
self refers to the instance

WORKED EXAMPLES

See it step by step

Read the code, predict the output, then compare it with the result.

01. Build and describe an object

PYTHON

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

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

pet = Dog("Pip")
print(pet.describe())

EXPECTED OUTPUT

Dog: Pip

__init__ stores the argument on this object. The method reads that stored attribute through self.

02. Keep objects independent

PYTHON

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

first = Counter()
second = Counter()
first.value = 2
print(first.value)
print(second.value)

EXPECTED OUTPUT

2
0

Each new Counter has its own value. Changing first does not change second.

HANDS-ON

Try it in Python

Edit the example and run it. Python starts in your browser the first time you click Run.

Python console

Ready to run

class Book:
    def __init__(self, title, pages):
        self.title = title
        self.pages = pages

    def summary(self):
        return f"{self.title}: {self.pages} pages"

first_book = Book("Python Notes", 80)
print(first_book.summary())

Need input()? Add one value per line

Input lines

Ctrl / ⌘ + Enter to run

OUTPUT

Your output appears here.

Your turnCreate a second Book named 'Code Journal' with 120 pages and store it in second_book. Print its summary, then print the first book summary again.

Need a hint?

Construct second_book = Book('Code Journal', 120). Call second_book.summary() inside print(), then call first_book.summary() again.

Complete the task and select Check task to verify your code.

KNOWLEDGE CHECK

Quick quiz

Three questions. You can change your answers and try again.

DEBUG BETTER

Typical mistakes

Everyone meets these errors. See what causes them and how to fix them.

Omitting self in a method

COMMON MISTAKE

class Book:
    def summary():
        return "A book"
book = Book()
print(book.summary())

FIX

class Book:
    def summary(self):
        return "A book"
book = Book()
print(book.summary())

What happens: TypeError because Python passes the instance automatically.
Instance methods need self as their first parameter.

Forgetting self on an attribute