OOP — Instance Methods & State

Instance methods and state

Instance methods use self to access and change object data.

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

    def increment(self):
        self.value += 1

Reading state with methods

Methods can return values.

python
class BankAccount:
    def __init__(self, balance):
        self.balance = balance

    def get_balance(self):
        return self.balance

Updating state safely

Methods can also modify the state, like deposits and withdrawals.

python
class BankAccount:
    def __init__(self, balance):
        self.balance = balance

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

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

Multiple objects

Each object keeps its own state.

python
a = BankAccount(100)
b = BankAccount(50)
a.deposit(25)
print(a.get_balance())
print(b.get_balance())

Common mistakes to avoid

• Forgetting to use self inside methods.
• Changing state directly from outside instead of using methods.
• Sharing one object when you actually need two.

What you should understand after this lesson

• How instance methods read and update object state.
• Why self is required.
• How objects keep data separate from each other.