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OOP – Practical Application

A class models a real-world entity by combining related attributes and methods in one structure

What it does: __init__ runs automatically when a new object is created. Methods can read and modify the object's own attributes via self. OOP keeps the data (balance) and the operations (deposit, withdraw) in one place. __init__ stores the initial state — every account needs an owner and a starting balance. Good OOP enforces its own rules — the withdraw method guards against going negative. deposit() modifies the object's own balance using self.balance += amount. Each BankAccount() call creates a separate account with its own balance. Encapsulation keeps data and the code that manipulates it together in one class.

Common mistakes: No balance check could go negative. Add validation. Must use self.balance balance alone is undefined here.

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Learning

Duo

Why use OOP for a bank account?

Groups related data and behavior togetherIt's faster

Matches

❌ def __init__(name):

↓

Error: Missing self in __init__

Applying

Meanings

Term

Encapsulation

Meaning

Bundling data and methods that operate on it together in a class.

Practice

class BankAccount:

Testing

Gaps

self. += amount

Check Answer

Spots

2 def withdraw(self, amount):

3 self.balance -= amount

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Quick Questions

1. Why use OOP for a bank account?

A. Groups related data and behavior together ✓

B. It's faster

OOP keeps the data (balance) and the operations (deposit, withdraw) in one place

2. What should __init__ set for a BankAccount?

A. Only the class name

B. owner and balance ✓

__init__ stores the initial state — every account needs an owner and a starting balance

3. Why check balance before withdrawing?

A. To prevent overdraft ✓

B. Python requires it

Good OOP enforces its own rules — the withdraw method guards against going negative

4. acc.deposit(50) what should this do?

A. Create a new account

B. Add 50 to self.balance ✓

deposit() modifies the object's own balance using self.balance += amount

5. Can one class have many objects?

A. Yes each with their own data ✓

B. No only one

Each BankAccount() call creates a separate account with its own balance

Match the Pairs

def __init__(name): → Error: Missing self in __init__

acc.balance() → Error: balance is an attribute, not a method

def __init__(self, name): → Valid: Constructor with self and a parameter

self.balance += amount → Ok: Update an instance attribute in a method

round(3.14159, 2) → Output: 3.14

abs(-7) → Output: 7

Key Term

Encapsulation

Bundling data and methods that operate on it together in a class.~~State

Fill in the Blanks

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

2. class TodoList: def add(self, task): self.tasks.append(task)

3. class BankAccount: def withdraw(self, amount): if amount <= self.balance: self.balance -= amount

4. acc = BankAccount('Alice', 100) acc.deposit(50)

5. class Score: def __init__(self): self.points = 0

Spot the Error

Question 1

A class BankAccount:

B def withdraw(self, amount):

C self.balance -= amount

Line C — No balance check could go negative. Add validation.

Question 2

A class BankAccount:

B def deposit(self, amount):

C balance += amount

Line C — Must use self.balance balance alone is undefined here

Question 3

A class TodoList:

B def __init__(self):

C tasks = []

D def add(self, task):

E self.tasks.append(task)

Line C — tasks = [] is a local variable. Use self.tasks = [] to store on object.

Question 4

A class BankAccount:

B def deposit(amount):

C self.balance += amount

Line B — Missing self as first parameter

Question 5

A class BankAccount:

B def __init__(self):

C self.balance = 0

D def deposit(self, amount):

E self.balance += amount

Line E — No validation negative deposits shouldn't be allowed

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