Encapsulation

Encapsulation means an object controls access to its own data, instead of letting any code reach in and set it to something invalid.

class BankAccount:
    def __init__(self, balance):
        self._balance = balance  # convention: "protected" by leading underscore

    def deposit(self, amount):
        if amount <= 0:
            raise ValueError("Deposit must be positive")
        self._balance += amount

    @property
    def balance(self):
        return self._balance

Python doesn't enforce privacy the way Java does — the leading underscore is a convention, not a lock. The @property decorator lets you expose account.balance as read-only, computed from a method, without the caller knowing it's not a plain attribute.