Instance attributes in Python - Mini Project: Build & Apply

Instance Attributes

📖 Scenario: You are creating a simple program to store information about books in a library. Each book has a title and an author.

🎯 Goal: Build a Python class called Book that uses instance attributes to store the title and author of each book. Then create one book object and print its details.

📋 What You'll Learn

Create a class named Book

Add an __init__ method with parameters self, title, and author

Inside __init__, set instance attributes self.title and self.author

Create an instance of Book named my_book with title '1984' and author 'George Orwell'

Print the title and author of my_book in the format: Title: 1984, Author: George Orwell

💡 Why This Matters

🌍 Real World

Storing information about objects like books, users, or products is common in software. Instance attributes let each object keep its own data.

💼 Career

Understanding instance attributes is essential for working with classes and objects in Python, a key skill for many programming jobs.

1

Create the Book class with an __init__ method

Create a class called Book with an __init__ method that takes self, title, and author as parameters. Inside __init__, set the instance attributes self.title and self.author to the values of title and author respectively.

Python

# Your code here

Hint

Remember, self refers to the current object. Use it to store the title and author as instance attributes.

2

Create an instance of Book

Create an instance of the Book class named my_book with the title '1984' and author 'George Orwell'.

Python

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

# Your code here

Hint

Use the class name Book followed by parentheses with the title and author as arguments.

3

Access instance attributes

Use my_book.title and my_book.author to access the title and author of the book.

Python

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

my_book = Book('1984', 'George Orwell')

# Your code here

Hint

Use dot notation to get the values stored in the instance attributes.

4

Print the book details

Print the title and author of my_book in the format: Title: 1984, Author: George Orwell using an f-string.

Python

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

my_book = Book('1984', 'George Orwell')

title = my_book.title
author = my_book.author

# Your code here

Hint

Use print(f"Title: {title}, Author: {author}") to display the book details.

Practice

1. What is an instance attribute in Python classes?

easy

A. A variable shared by all objects of the class

B. A function that belongs to the class

C. A method to create new objects

D. A variable that stores data unique to each object

Solution

1. Step 1: Understand instance attributes

   Instance attributes are variables that belong to each object separately, not shared.

2. Step 2: Differentiate from class attributes

   Class attributes are shared by all objects, but instance attributes hold unique data per object.

3. Final Answer:

   A variable that stores data unique to each object -> Option D

4. Quick Check:

   Instance attribute = unique data per object [OK]

Hint: Instance attributes belong to objects, not the class itself [OK]

Common Mistakes:

• Confusing instance attributes with class attributes
• Thinking methods are attributes
• Assuming all objects share the same attribute values

2. Which of the following is the correct way to define an instance attribute inside a class?

easy

A. name = "Alice" outside any method

B. def name(self): return "Alice"

C. self.name = "Alice" inside __init__ method

D. class.name = "Alice" inside __init__

Solution

1. Step 1: Identify instance attribute syntax

   Instance attributes are set inside __init__ using self.attribute = value.

2. Step 2: Check each option

   self.name = "Alice" inside __init__ method uses self.name = "Alice" inside __init__, which is correct. Others are class attributes, methods, or invalid.

3. Final Answer:

   self.name = "Alice" inside __init__ method -> Option C

4. Quick Check:

   Instance attribute = self.attribute inside __init__ [OK]

Hint: Use self.attribute = value inside __init__ for instance attributes [OK]

Common Mistakes:

• Defining attributes outside __init__ without self
• Using class.attribute instead of self.attribute
• Confusing methods with attributes

3. What will be the output of this code?

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

dog1 = Dog("Buddy")
dog2 = Dog("Max")
print(dog1.name)
print(dog2.name)

medium

A. Buddy
Max

B. Max
Buddy

C. Buddy
Buddy

D. Max
Max

Solution

1. Step 1: Understand instance attribute assignment

   dog1.name is set to "Buddy" and dog2.name is set to "Max" separately.

2. Step 2: Print instance attributes

   Printing dog1.name outputs "Buddy" and dog2.name outputs "Max".

3. Final Answer:

   Buddy Max -> Option A

4. Quick Check:

   Each object has its own name attribute [OK]

Hint: Each object keeps its own attribute values [OK]

Common Mistakes:

• Assuming all objects share the same attribute
• Mixing up the order of print statements
• Confusing class and instance attributes

4. Find the error in this code:

class Car:
    def __init__(self, model):
        model = model

car = Car("Tesla")
print(car.model)

medium

A. AttributeError because model is not set as instance attribute

B. SyntaxError due to missing self

C. TypeError because __init__ has wrong parameters

D. No error, prints Tesla

Solution

1. Step 1: Check attribute assignment in __init__

   The code assigns model = model, which only assigns local variable, not instance attribute.

2. Step 2: Accessing car.model causes error

   Since self.model is never set, car.model does not exist, causing AttributeError.

3. Final Answer:

   AttributeError because model is not set as instance attribute -> Option A

4. Quick Check:

   Use self.model = model to set instance attribute [OK]

Hint: Always use self.attribute = value to set instance attributes [OK]

Common Mistakes:

• Forgetting self. when assigning attributes
• Assuming local variable sets instance attribute
• Expecting attribute to exist without self

5. You want to create a class Book where each book has a title and a list of authors. How do you correctly set instance attributes so each book has its own authors list without sharing it between objects?

hard

A. Set self.authors = None and assign list later

B. Set self.authors = [] inside __init__ method

C. Set self.authors = authors where authors is a default empty list in parameters

D. Set authors = [] as a class attribute outside methods

Solution

1. Step 1: Avoid shared mutable class attributes

   Setting authors = [] as class attribute shares the same list across all objects, causing bugs.

2. Step 2: Initialize instance attribute inside __init__

   Setting self.authors = [] inside __init__ creates a new list for each object, avoiding sharing.

3. Final Answer:

   Set self.authors = [] inside __init__ method -> Option B

4. Quick Check:

   Mutable instance attributes must be set inside __init__ [OK]

Hint: Initialize mutable attributes inside __init__ to avoid sharing [OK]

Common Mistakes:

• Using mutable default arguments in method parameters
• Defining mutable attributes as class variables
• Not initializing mutable attributes per instance