Adding custom attributes in Python - Mini Project: Build & Apply

Adding Custom Attributes to a Python Object

📖 Scenario: You are creating a simple program to store information about a book. You want to add extra details to the book object by adding custom attributes.

🎯 Goal: Build a Python program that creates a book object, adds a custom attribute to it, and then prints that attribute.

📋 What You'll Learn

Create a class called Book with a title attribute

Create an instance of Book called my_book with the title 'Python Basics'

Add a custom attribute called author to my_book with the value 'Alice'

Print the value of the author attribute from my_book

💡 Why This Matters

🌍 Real World

Adding custom attributes to objects is useful when you want to store extra information that was not planned when the class was first created.

💼 Career

Understanding how to add and use custom attributes helps in flexible software design and is common in many programming jobs involving object-oriented programming.

1

Create the Book class and an instance

Create a class called Book with an __init__ method that takes title as a parameter and sets it as an attribute. Then create an instance called my_book with the title 'Python Basics'.

Python

# Create the Book class and an instance called my_book
# Your code here

Hint

Use class Book: to start the class. Inside, define def __init__(self, title): and set self.title = title. Then create my_book = Book('Python Basics').

2

Add a custom attribute to the book instance

Add a custom attribute called author to the my_book object and set its value to 'Alice'.

Python

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

my_book = Book('Python Basics')
# Add the author attribute to my_book
# Your code here

Hint

Use dot notation to add the attribute: my_book.author = 'Alice'.

3

Access the custom attribute

Write a line of code that accesses the author attribute from my_book and stores it in a variable called book_author.

Python

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

my_book = Book('Python Basics')
my_book.author = 'Alice'
# Store the author attribute in book_author
# Your code here

Hint

Use book_author = my_book.author to get the author attribute.

4

Print the custom attribute

Print the value of the variable book_author.

Python

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

my_book = Book('Python Basics')
my_book.author = 'Alice'
book_author = my_book.author
# Print book_author
# Your code here

Hint

Use print(book_author) to show the author.

Practice

1. What does adding a custom attribute to a Python object allow you to do?

easy

A. Delete the object from memory

B. Change the object's type permanently

C. Store extra information directly on that object

D. Prevent the object from being used in functions

Solution

1. Step 1: Understand what attributes are

   Attributes are values or properties stored inside an object to hold data or state.

2. Step 2: Adding custom attributes

   When you add a custom attribute, you attach new data directly to that object, allowing it to hold extra information.

3. Final Answer:

   Store extra information directly on that object -> Option C

4. Quick Check:

   Custom attributes = extra data on object [OK]

Hint: Custom attributes add new data fields to objects [OK]

Common Mistakes:

• Thinking attributes change the object's type
• Confusing attributes with deleting objects
• Believing attributes restrict object usage

2. Which of the following is the correct syntax to add a custom attribute color with value 'red' to an object car?

easy

A. car.color = 'red'

B. car['color'] = 'red'

C. car->color = 'red'

D. car.color('red')

Solution

1. Step 1: Identify attribute assignment syntax

   In Python, attributes are assigned using dot notation: object.attribute = value.

2. Step 2: Check each option

   car.color = 'red' uses dot notation correctly. car['color'] = 'red' uses dictionary syntax which is invalid for normal objects. car->color = 'red' uses arrow notation which is not Python syntax. car.color('red') tries to call attribute as a method, which is incorrect here.

3. Final Answer:

   car.color = 'red' -> Option A

4. Quick Check:

   Use dot notation to add attributes [OK]

Hint: Use dot notation: object.attribute = value [OK]

Common Mistakes:

• Using dictionary syntax on objects
• Using arrow (->) like in other languages
• Calling attribute as a function

3. What will be the output of this code?

class Dog:
    pass

my_dog = Dog()
my_dog.age = 5
print(my_dog.age)

medium

A. AttributeError

B. age

C. Dog

D. 5

Solution

1. Step 1: Understand attribute assignment

   The code creates an empty class Dog and an instance my_dog. Then it adds a custom attribute age with value 5 to my_dog.

2. Step 2: Print the attribute value

   Printing my_dog.age outputs the value stored, which is 5.

3. Final Answer:

   5 -> Option D

4. Quick Check:

   Custom attribute value prints correctly [OK]

Hint: Print attribute after assignment to see its value [OK]

Common Mistakes:

• Expecting class name or attribute name as output
• Thinking attribute does not exist yet
• Confusing attribute with method call

4. Find the error in this code that tries to add a custom attribute height to an object person:

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

person = Person('Alice')
person.height = 170
print(person['height'])

medium

A. Missing height parameter in __init__

B. Using person['height'] instead of person.height

C. Cannot add attributes after object creation

D. Name attribute should be height

Solution

1. Step 1: Check how attribute is added

   The code correctly adds height using dot notation: person.height = 170.

2. Step 2: Identify incorrect attribute access

   Printing uses person['height'], which is dictionary syntax and invalid for normal objects. It should be person.height.

3. Final Answer:

   Using person['height'] instead of person.height -> Option B

4. Quick Check:

   Access attributes with dot, not brackets [OK]

Hint: Use dot notation to access attributes, not brackets [OK]

Common Mistakes:

• Using dictionary syntax on objects
• Thinking attributes must be in __init__
• Believing attributes can't be added later

5. You want to add a custom attribute status to multiple instances of a class Task only if their priority is above 5. Which code correctly does this?

hard

A. for t in tasks:
if t.priority > 5:
t.status = 'urgent'

B. for t in tasks:
t.status = 'urgent' if t.priority > 5 else None

C. for t in tasks:
if t.priority > 5:
t['status'] = 'urgent'

D. for t in tasks:
t.status = 'urgent'
if t.priority <= 5:
del t.status

Solution

1. Step 1: Understand the condition and attribute addition

   We want to add status only if priority > 5, so we check this condition inside the loop.

2. Step 2: Check each option's logic and syntax

   for t in tasks: if t.priority > 5: t.status = 'urgent' adds status only when priority > 5 using dot notation correctly. for t in tasks: t.status = 'urgent' if t.priority > 5 else None assigns status to None for others, which adds the attribute unnecessarily. for t in tasks: if t.priority > 5: t['status'] = 'urgent' uses dictionary syntax which is invalid. for t in tasks: t.status = 'urgent' if t.priority <= 5: del t.status adds status to all then deletes for low priority, which is inefficient and error-prone.

3. Final Answer:

   for t in tasks: if t.priority > 5: t.status = 'urgent' -> Option A

4. Quick Check:

   Use if condition and dot notation to add attributes selectively [OK]

Hint: Add attributes inside if-block with dot notation [OK]

Common Mistakes:

• Using dictionary syntax on objects
• Adding attribute to all instances regardless of condition
• Assigning None instead of skipping attribute