Single Responsibility and Open/Closed Principles

# --- BEFORE: violates SRP and OCP ---

class Invoice:
    def __init__(self, items, discount_type):
        self.items = items          # list of (name, price)
        self.discount_type = discount_type

    def total(self):
        subtotal = sum(price for _, price in self.items)
        # OCP violation: adding a new discount type means editing this method
        if self.discount_type == "none":
            return subtotal
        elif self.discount_type == "seasonal":
            return subtotal * 0.9
        elif self.discount_type == "loyalty":
            return subtotal * 0.8
        else:
            raise ValueError("unknown discount type")

    def to_html(self):
        # SRP violation: this class also knows about presentation
        rows = "".join(f"<tr><td>{n}</td><td>{p}</td></tr>" for n, p in self.items)
        return f"<table>{rows}</table><p>Total: {self.total()}</p>"


# --- AFTER: SRP-compliant split + OCP-compliant extension point ---

from abc import ABC, abstractmethod

class DiscountPolicy(ABC):
    @abstractmethod
    def apply(self, subtotal: float) -> float:
        ...

class NoDiscount(DiscountPolicy):
    def apply(self, subtotal: float) -> float:
        return subtotal

class SeasonalDiscount(DiscountPolicy):
    def apply(self, subtotal: float) -> float:
        return subtotal * 0.9

class LoyaltyDiscount(DiscountPolicy):
    def apply(self, subtotal: float) -> float:
        return subtotal * 0.8

# New discount types are added by creating new classes -- no edits below.

class Invoice:
    def __init__(self, items, discount_policy: DiscountPolicy):
        self.items = items
        self.discount_policy = discount_policy

    def total(self) -> float:
        subtotal = sum(price for _, price in self.items)
        return self.discount_policy.apply(subtotal)


class InvoiceHtmlFormatter:
    def render(self, invoice: Invoice) -> str:
        rows = "".join(f"<tr><td>{n}</td><td>{p}</td></tr>" for n, p in invoice.items)
        return f"<table>{rows}</table><p>Total: {invoice.total()}</p>"