Personal Finance Manager

import json
import os
from datetime import date
from functools import lru_cache

class Transaction:
    VALID_TYPES = ("income", "expense")

    def __init__(self, date_str, type_, category, amount, description=""):
        if type_ not in self.VALID_TYPES:
            raise ValueError(f"type must be one of {self.VALID_TYPES}")
        if amount <= 0:
            raise ValueError("amount must be positive")
        self.date = date_str
        self.type = type_
        self.category = category
        self.amount = float(amount)
        self.description = description

    def to_dict(self):
        return {
            "date": self.date,
            "type": self.type,
            "category": self.category,
            "amount": self.amount,
            "description": self.description,
        }

    @classmethod
    def from_dict(cls, data):
        return cls(data["date"], data["type"], data["category"], data["amount"], data.get("description", ""))


class FinanceManager:
    def __init__(self):
        self.transactions = []
        self._report_cache = {}

    def add_transaction(self, transaction):
        self.transactions.append(transaction)
        self._report_cache.clear()  # invalidate cache when data changes

    def get_balance(self):
        income = sum(t.amount for t in self.transactions if t.type == "income")
        expenses = sum(t.amount for t in self.transactions if t.type == "expense")
        return income - expenses

    def get_report(self, month):
        """month format: YYYY-MM. Cached until transactions change."""
        if month in self._report_cache:
            return self._report_cache[month]

        income_total = 0.0
        expense_total = 0.0
        categories = {}

        for t in self.transactions:
            if not t.date.startswith(month):
                continue
            if t.type == "income":
                income_total += t.amount
            else:
                expense_total += t.amount
                categories[t.category] = categories.get(t.category, 0) + t.amount

        report = {
            "month": month,
            "income": income_total,
            "expenses": expense_total,
            "net": income_total - expense_total,
            "by_category": categories,
        }
        self._report_cache[month] = report
        return report

    def save_to_file(self, filename="finance.json"):
        with open(filename, "w") as f:
            json.dump([t.to_dict() for t in self.transactions], f, indent=2)
        print("Data saved.")

    def load_from_file(self, filename="finance.json"):
        if not os.path.exists(filename):
            return
        with open(filename, "r") as f:
            data = json.load(f)
        self.transactions = [Transaction.from_dict(d) for d in data]
        self._report_cache.clear()


def main():
    manager = FinanceManager()
    manager.load_from_file()

    while True:
        print("\n--- Personal Finance Manager ---")
        print("1. Add Income")
        print("2. Add Expense")
        print("3. View Balance")
        print("4. Monthly Report")
        print("5. Save & Exit")
        choice = input("Choose an option: ")

        if choice in ("1", "2"):
            type_ = "income" if choice == "1" else "expense"
            category = input("Category: ")
            try:
                amount = float(input("Amount: "))
                description = input("Description: ")
                t = Transaction(str(date.today()), type_, category, amount, description)
                manager.add_transaction(t)
                print("Transaction added.")
            except ValueError as e:
                print(f"Error: {e}")
        elif choice == "3":
            print(f"Current balance: ${manager.get_balance():.2f}")
        elif choice == "4":
            month = input("Enter month (YYYY-MM): ")
            report = manager.get_report(month)
            print(f"\nReport for {report['month']}:")
            print(f"  Income:   ${report['income']:.2f}")
            print(f"  Expenses: ${report['expenses']:.2f}")
            print(f"  Net:      ${report['net']:.2f}")
            print("  By category:")
            for cat, amt in report["by_category"].items():
                print(f"    {cat}: ${amt:.2f}")
        elif choice == "5":
            manager.save_to_file()
            print("Goodbye!")
            break
        else:
            print("Invalid option.")

if __name__ == "__main__":
    main()