Python Classes/Objects

Python OOP: Classes, Objects, & Dunder Methods

Object-Oriented Programming (OOP) in Python models concepts as classes with encapsulated attributes and methods. Python uses double-underscore magic methods (dunder methods) like __init__, __str__, and __repr__ to integrate objects into core language protocols.

Class Architecture

flowchart TD
    A["Class Definition (BankAccount)"] --> B["Class Attribute (bank_name)"]
    A --> C["Instance Attributes (account_id, balance)"]
    A --> D["Dunder Methods (__init__, __repr__, __eq__)"]
    A --> E["Instance Methods (deposit, withdraw)"]

Key Dunder Methods

• __init__(self, ...): Instance initializer constructor.
• __repr__(self): Unambiguous developer string representation for debugging (repr(obj)).
• __str__(self): Human-readable string representation (str(obj) / print(obj)).
• __eq__(self, other): Equality operator overloading (==).

Practical Code Example

class BankAccount:
    """Represent a customer bank account with balance guards."""
    bank_name: str = "Global Tech Bank"  # Class Attribute

    def __init__(self, account_id: str, initial_balance: float = 0.0) -> None:
        if initial_balance < 0:
            raise ValueError("Initial balance cannot be negative.")
        self.account_id: str = account_id       # Instance Attribute
        self._balance: float = initial_balance  # Protected Instance Attribute

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

    def withdraw(self, amount: float) -> float:
        if amount > self._balance:
            raise ValueError("Insufficient funds for withdrawal.")
        self._balance -= amount
        return self._balance

    def __repr__(self) -> str:
        return f"BankAccount(account_id={self.account_id!r}, balance={self._balance})"

    def __str__(self) -> str:
        return f"Account #{self.account_id} | Balance: ${self._balance:.2f}"

if __name__ == "__main__":
    acc = BankAccount("ACC-1092", 500.0)
    acc.deposit(250.0)
    print("Developer Repr:", repr(acc))
    print("User String:", str(acc))

Best Practices & Gotchas

• Always Implement __repr__: If you only implement one representation method, implement __repr__ because __str__ defaults to __repr__ if omitted.
• The self Parameter: self is an explicit reference to the current object instance passed automatically by Python during method invocation.
• Use Encapsulation Naming Conventions: Use a single leading underscore (_protected) to signal internal attributes, or double underscores (__private) for name-mangling.

Self-Check Challenge

Create a class Rectangle with attributes width and height, a method area() -> float, and a __str__ method returning "Rectangle WxH".