Encapsulation

Encapsulation means keeping an object's internal details private, and only exposing a deliberate, controlled way to interact with it. Python signals "this is internal, don't touch it directly" with a leading underscore, by convention rather than strict enforcement.

class BankAccount:
    def __init__(self, balance):
        self._balance = balance   # "_" signals: internal use only

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

    def balance(self):
        return self._balance

Code outside the class calls account.deposit(50) rather than directly changing account._balance = -1000000, so the validation inside deposit() can't be bypassed.

Common mistakes

• Relying on the underscore as real security — it's only a convention. Python doesn't stop external code from touching _balance directly; it just signals that doing so isn't supported.