Day 30: OOP: Classes and Objects

Quick Answer

What You Will Learn

• class BankAccount: defines a blueprint. bank is a class variable, shared by all accounts.
• __init__(self, owner, balance=0) runs when a new account is created and sets owner and balance.
• self is the object the method was called on. Forgetting self in a method raises a TypeError.
• deposit() increases the balance and raises ValueError for a zero or negative amount.
• @property balance lets balance be read like an attribute, but not set directly (a read-only getter).
• The four pillars of OOP are Encapsulation, Abstraction, Inheritance and Polymorphism, which come in the following days.

Code

class BankAccount:
    bank = "SBI"

    def __init__(self, owner, balance=0):
        self.owner = owner
        self._balance = balance

    def deposit(self, amt):
        if amt <= 0:
            raise ValueError("Amount must be > 0")
        self._balance += amt

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

acc = BankAccount("Arhan")
acc.deposit(500)
print(acc.balance, acc.bank)
print(acc.owner)

Output

500 SBI
Arhan

Code Explained, Line by Line

1. class BankAccount: defines a blueprint. bank is a class variable, shared by all accounts.
2. __init__(self, owner, balance=0) runs when a new account is created and sets owner and balance.
3. self is the object the method was called on. Forgetting self in a method raises a TypeError.
4. deposit() increases the balance and raises ValueError for a zero or negative amount.
5. @property balance lets balance be read like an attribute, but not set directly (a read-only getter).
6. The four pillars of OOP are Encapsulation, Abstraction, Inheritance and Polymorphism, which come in the following days.

Real-Life Use

Bank accounts, users, orders and products in real apps are all classes with objects.

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