OOP — এনক্যাপসুলেশন আর ডান্ডার মেথড

class BankAccount:
    def __init__(self):
        self._balance = 0  # single underscore: "internal, please don't touch"

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

    def get_balance(self):
        return self._balance

account = BankAccount()
account.deposit(500)
print(account.get_balance())  # 500
print(account._balance)       # 500 — this works! Python doesn't block it, even though the underscore says not to

class Student:
    def __init__(self, name):
        self.name = name

    def __str__(self):
        return f"Student: {self.name}"

s = Student("Priya")
print(s)         # "Student: Priya" — print() calls __str__ automatically
print(str(s))    # same thing

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

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

    @balance.setter
    def balance(self, value):
        if value < 0:
            raise ValueError("Balance cannot be negative.")
        self._balance = value

account = BankAccount()
account.balance = 500   # looks like a plain attribute, actually calls the setter
print(account.balance)  # 500 — looks like a plain attribute, actually calls the getter
# account.balance = -10 # ValueError — the setter blocks this