Python Inheritance Explained — How, Why, and When to Use It

# Real-world example: a generic BankAccount and a specialised SavingsAccount

class BankAccount:
    """The parent class — holds logic every bank account needs."""

    def __init__(self, owner: str, balance: float = 0.0):
        self.owner = owner
        self.balance = balance  # shared attribute every account type needs

    def deposit(self, amount: float) -> None:
        """Add funds. Validation lives here once, not in every subclass."""
        if amount <= 0:
            raise ValueError("Deposit amount must be positive.")
        self.balance += amount
        print(f"Deposited £{amount:.2f}. New balance: £{self.balance:.2f}")

    def withdraw(self, amount: float) -> None:
        """Base withdrawal — no special rules yet."""
        if amount > self.balance:
            raise ValueError("Insufficient funds.")
        self.balance -= amount
        print(f"Withdrew £{amount:.2f}. Remaining: £{self.balance:.2f}")

    def __repr__(self) -> str:
        return f"BankAccount(owner='{self.owner}', balance=£{self.balance:.2f})"


class SavingsAccount(BankAccount):  # <-- inherits from BankAccount
    """Child class — adds an interest rate and enforces a minimum balance."""

    MINIMUM_BALANCE = 100.0  # class-level rule specific to savings accounts

    def __init__(self, owner: str, balance: float = 0.0, interest_rate: float = 0.03):
        # super().__init__ calls BankAccount.__init__ so we don't duplicate that logic
        super().__init__(owner, balance)
        self.interest_rate = interest_rate  # SavingsAccount-specific attribute

    def withdraw(self, amount: float) -> None:
        """Override parent's withdraw to enforce minimum balance rule."""
        if (self.balance - amount) < self.MINIMUM_BALANCE:
            raise ValueError(
                f"Cannot go below minimum balance of £{self.MINIMUM_BALANCE:.2f}."
            )
        # Delegate the actual withdrawal logic back to the parent — DRY!
        super().withdraw(amount)

    def apply_interest(self) -> None:
        """New method that only SavingsAccount has — not on the parent."""
        interest_earned = self.balance * self.interest_rate
        self.balance += interest_earned
        print(f"Interest applied: £{interest_earned:.2f}. Balance: £{self.balance:.2f}")

    def __repr__(self) -> str:
        return (
            f"SavingsAccount(owner='{self.owner}', "
            f"balance=£{self.balance:.2f}, rate={self.interest_rate:.1%})"
        )


# --- Usage ---
account = SavingsAccount(owner="Alice", balance=500.0, interest_rate=0.05)
print(account)

account.deposit(200.0)        # inherited from BankAccount — no code duplication
account.apply_interest()      # only available on SavingsAccount
account.withdraw(100.0)       # overridden version — checks minimum balance

try:
    account.withdraw(560.0)   # this should fail — would breach minimum balance
except ValueError as error:
    print(f"Blocked: {error}")

print(account)

# Multilevel inheritance: Vehicle -> Car -> ElectricCar
# Multiple inheritance via a Mixin: adding GPS capability cleanly

class Vehicle:
    """Top of the chain — every vehicle has these basics."""

    def __init__(self, make: str, model: str, year: int):
        self.make = make
        self.model = model
        self.year = year

    def start_engine(self) -> str:
        return f"{self.make} {self.model} engine started."

    def __repr__(self) -> str:
        return f"{self.year} {self.make} {self.model}"


class Car(Vehicle):
    """Multilevel level 2 — a Car IS-A Vehicle with door-count logic."""

    def __init__(self, make: str, model: str, year: int, num_doors: int = 4):
        super().__init__(make, model, year)  # pass shared args up the chain
        self.num_doors = num_doors

    def honk(self) -> str:
        return "Beep beep!"


class GpsNavigationMixin:
    """
    A mixin — NOT a standalone class, no __init__ of its own.
    It adds GPS behaviour to any class that includes it.
    """

    def get_current_location(self) -> str:
        # In a real app this would call a GPS API
        return "51.5074° N, 0.1278° W (London, UK)"

    def navigate_to(self, destination: str) -> str:
        return f"Navigating to '{destination}'... Turn right in 200m."


class ElectricCar(GpsNavigationMixin, Car):
    """
    Multilevel level 3 + Multiple inheritance.
    ElectricCar IS-A Car, and also HAS GPS navigation (via mixin).
    MRO: ElectricCar -> GpsNavigationMixin -> Car -> Vehicle -> object
    """

    def __init__(self, make: str, model: str, year: int, battery_kwh: float):
        # super().__init__ here follows the MRO — it reaches Car correctly
        super().__init__(make, model, year)
        self.battery_kwh = battery_kwh
        self.charge_level = 100.0  # percentage

    def start_engine(self) -> str:
        # Override parent's method — electric cars don't have a combustion engine
        return f"{self.make} {self.model} motor silently activated. ⚡"

    def charge_status(self) -> str:
        return f"Battery: {self.charge_level:.0f}% ({self.battery_kwh} kWh capacity)"


# --- Inspect the MRO before using it ---
print("MRO:", [cls.__name__ for cls in ElectricCar.__mro__])
print()

tesla = ElectricCar(make="Tesla", model="Model 3", year=2024, battery_kwh=82.0)
print(tesla)                          # from Vehicle.__repr__
print(tesla.start_engine())           # overridden in ElectricCar
print(tesla.honk())                   # inherited from Car
print(tesla.charge_status())          # ElectricCar's own method
print(tesla.get_current_location())   # from GpsNavigationMixin
print(tesla.navigate_to("Heathrow"))  # from GpsNavigationMixin

from abc import ABC, abstractmethod
from datetime import datetime


class PaymentProcessor(ABC):
    """
    Abstract base class — defines the CONTRACT for all payment processors.
    You cannot instantiate this class directly. Any subclass MUST implement
    the abstract methods or Python raises TypeError at class creation time.
    """

    def __init__(self, merchant_id: str):
        self.merchant_id = merchant_id
        self._transaction_log: list = []

    @abstractmethod
    def process_payment(self, amount: float, currency: str) -> dict:
        """
        Every payment processor must define HOW it handles a payment.
        The what (a payment happens) is the contract. The how is up to each subclass.
        """
        pass  # abstract — no body needed

    @abstractmethod
    def refund(self, transaction_id: str) -> bool:
        """Every processor must support refunds."""
        pass

    # Concrete method — shared by ALL subclasses, no override needed
    def log_transaction(self, transaction_id: str, amount: float, status: str) -> None:
        """Non-abstract: logging works the same for every processor."""
        entry = {
            "id": transaction_id,
            "amount": amount,
            "status": status,
            "timestamp": datetime.now().strftime("%Y-%m-%d %H:%M:%S"),
        }
        self._transaction_log.append(entry)
        print(f"[LOG] Transaction {transaction_id}: {status} — £{amount:.2f}")

    def get_transaction_history(self) -> list:
        return self._transaction_log


class StripeProcessor(PaymentProcessor):
    """Concrete implementation — fulfils the PaymentProcessor contract via Stripe."""

    def process_payment(self, amount: float, currency: str) -> dict:
        # In reality: call stripe.PaymentIntent.create(...)
        transaction_id = f"stripe_txn_{int(datetime.now().timestamp())}"
        print(f"Stripe: charging £{amount:.2f} {currency} via card on file...")
        self.log_transaction(transaction_id, amount, "SUCCESS")
        return {"processor": "Stripe", "transaction_id": transaction_id, "status": "SUCCESS"}

    def refund(self, transaction_id: str) -> bool:
        # In reality: call stripe.Refund.create(payment_intent=transaction_id)
        print(f"Stripe: processing refund for {transaction_id}...")
        return True


class PayPalProcessor(PaymentProcessor):
    """Different concrete implementation — same contract, completely different internals."""

    def __init__(self, merchant_id: str, client_id: str):
        super().__init__(merchant_id)      # call parent __init__ first
        self.client_id = client_id         # PayPal-specific credential

    def process_payment(self, amount: float, currency: str) -> dict:
        transaction_id = f"pp_txn_{int(datetime.now().timestamp())}"
        print(f"PayPal: initiating £{amount:.2f} {currency} order via REST API...")
        self.log_transaction(transaction_id, amount, "PENDING")
        return {"processor": "PayPal", "transaction_id": transaction_id, "status": "PENDING"}

    def refund(self, transaction_id: str) -> bool:
        print(f"PayPal: submitting refund request for {transaction_id}...")
        return True


# --- Prove the contract is enforced ---
try:
    bad_processor = PaymentProcessor(merchant_id="test")  # should fail
except TypeError as error:
    print(f"Caught expected error: {error}\n")

# --- Use the concrete classes polymorphically ---
processors: list[PaymentProcessor] = [
    StripeProcessor(merchant_id="merch_stripe_001"),
    PayPalProcessor(merchant_id="merch_pp_001", client_id="pp_client_abc"),
]

for processor in processors:
    result = processor.process_payment(amount=49.99, currency="GBP")
    print(f"Result: {result}\n")