Modifying object state in Python - Mini Project: Build & Apply

Modifying Object State in Python

📖 Scenario: You are creating a simple program to manage a bank account. The account has a balance that can change when you deposit or withdraw money.

🎯 Goal: Build a Python class called BankAccount that can store a balance and update it when deposits or withdrawals happen.

📋 What You'll Learn

Create a class named BankAccount with an attribute balance.

Add a method deposit that increases the balance by a given amount.

Add a method withdraw that decreases the balance by a given amount.

Print the final balance after some deposits and withdrawals.

💡 Why This Matters

🌍 Real World

Managing bank accounts or any system where you track and update values over time.

💼 Career

Understanding how to modify object state is key for software development, especially in building interactive applications and managing data.

1

Create the BankAccount class with initial balance

Create a class called BankAccount with an __init__ method that sets the balance attribute to 0.

Python

class BankAccount:
    def __init__(self):
        # Your code here

Hint

Use self.balance = 0 inside the __init__ method to set the starting balance.

2

Add a deposit method to increase balance

Add a method called deposit to the BankAccount class that takes a parameter amount and adds it to self.balance.

Python

class BankAccount:
    def __init__(self):
        self.balance = 0

    # Your code here

Hint

Use self.balance += amount inside the deposit method to add money.

3

Add a withdraw method to decrease balance

Add a method called withdraw to the BankAccount class that takes a parameter amount and subtracts it from self.balance.

Python

class BankAccount:
    def __init__(self):
        self.balance = 0

    def deposit(self, amount):
        self.balance += amount

    # Your code here

Hint

Use self.balance -= amount inside the withdraw method to remove money.

4

Create an account, deposit, withdraw, and print balance

Create an instance of BankAccount called account. Use the deposit method to add 100, then use the withdraw method to remove 30. Finally, print account.balance.

Python

class BankAccount:
    def __init__(self):
        self.balance = 0

    def deposit(self, amount):
        self.balance += amount

    def withdraw(self, amount):
        self.balance -= amount

# Your code here

Hint

Remember to create the object, call deposit(100), then withdraw(30), and print the balance.

Practice

1. What does it mean to modify the state of an object in Python?

easy

A. Changing the values of its attributes

B. Creating a new object

C. Deleting the object

D. Printing the object

Solution

1. Step 1: Understand object state

   The state of an object is stored in its attributes (variables inside the object).

2. Step 2: What modifying state means

   Modifying state means changing these attribute values to new ones.

3. Final Answer:

   Changing the values of its attributes -> Option A

4. Quick Check:

   Modify state = change attribute values [OK]

Hint: State means attribute values; changing them modifies state [OK]

Common Mistakes:

• Confusing creating a new object with modifying state
• Thinking printing changes state
• Mixing deleting object with modifying state

2. Which of the following is the correct way to change an object's attribute color to 'blue'?

easy

A. object.color('blue')

B. object->color = 'blue'

C. object.color = 'blue'

D. object[color] = 'blue'

Solution

1. Step 1: Attribute assignment syntax

   In Python, to change an attribute, use dot notation: object.attribute = value.

2. Step 2: Check each option

   object.color = 'blue' uses correct dot notation. Options A, B, and C use invalid syntax for attribute assignment.

3. Final Answer:

   object.color = 'blue' -> Option C

4. Quick Check:

   Use dot and = to assign attribute [OK]

Hint: Use dot and equals to set attribute: object.attr = value [OK]

Common Mistakes:

• Using brackets [] instead of dot for attributes
• Using arrow -> which is not Python syntax
• Trying to call attribute like a function

3. What will be the output of this code?

class Box:
    def __init__(self):
        self.size = 5

    def enlarge(self):
        self.size += 3

b = Box()
b.enlarge()
print(b.size)

medium

A. 5

B. 8

C. 3

D. Error

Solution

1. Step 1: Initial attribute value

   The Box object b starts with size = 5 from __init__.

2. Step 2: Method enlarge changes size

   Calling b.enlarge() adds 3 to size, so size becomes 5 + 3 = 8.

3. Final Answer:

   8 -> Option B

4. Quick Check:

   5 + 3 = 8 [OK]

Hint: Add changes inside method to attribute value [OK]

Common Mistakes:

• Forgetting method changes attribute
• Thinking size resets after method call
• Expecting error due to method call

4. Find the error in this code that tries to update an object's attribute:

class Car:
    def __init__(self):
        self.speed = 0

    def accelerate(self):
        speed += 10

c = Car()
c.accelerate()
print(c.speed)

medium

A. Using speed without self inside accelerate method

B. Missing self in accelerate method parameter

C. Incorrect print statement syntax

D. No error, code runs fine

Solution

1. Step 1: Check method parameter

   accelerate has self parameter, so it can access attributes.

2. Step 2: Identify attribute update

   Inside accelerate, speed += 10 tries to update speed but misses self. It should be self.speed += 10.

3. Final Answer:

   Using speed without self inside accelerate method -> Option A

4. Quick Check:

   Use self.speed to modify attribute [OK]

Hint: Always use self.attribute to change object state inside methods [OK]

Common Mistakes:

• Forgetting self. before attribute inside methods
• Thinking print syntax is wrong
• Assuming missing self parameter causes error here

5. You want to keep track of how many times a method use() is called on an object. Which is the best way to modify the object state to do this?

hard

A. Delete the object after each use() call

B. Print a message every time use() is called

C. Create a new object each time use() is called

D. Add an attribute count initialized to 0 and increase it inside use()

Solution

1. Step 1: Understand tracking calls

   To count calls, store a number in the object that updates each time.

2. Step 2: Modify state properly

   Initialize an attribute count = 0, then increase it by 1 inside use() method.

3. Final Answer:

   Add an attribute count initialized to 0 and increase it inside use() -> Option D

4. Quick Check:

   Use attribute to track count [OK]

Hint: Use attribute counter updated inside method to track calls [OK]

Common Mistakes:

• Just printing without storing count
• Creating new objects instead of updating state
• Deleting object removes all state