Compound interest calculator in Python

def future_value(
    principal: float,
    annual_rate: float,
    years: int,
    compounds_per_year: int = 12,
) -> float:
    """Return balance after compound interest (rounded to cents)."""
    rate_per_period = annual_rate / compounds_per_year
    periods = compounds_per_year * years
    amount = principal * (1 + rate_per_period) ** periods
    return round(amount, 2)


def yearly_balances(
    principal: float,
    annual_rate: float,
    years: int,
    compounds_per_year: int = 12,
) -> list[tuple[int, float]]:
    """Return (year, balance) after each full year."""
    schedule: list[tuple[int, float]] = []
    for year in range(1, years + 1):
        balance = future_value(principal, annual_rate, year, compounds_per_year)
        schedule.append((year, balance))
    return schedule


def main() -> None:
    principal = 1_000.0
    annual_rate = 0.05
    years = 10
    compounds_per_year = 12

    final = future_value(principal, annual_rate, years, compounds_per_year)
    print(f"Principal: ${principal:,.2f}")
    print(f"Rate: {annual_rate * 100:.1f}% compounded {compounds_per_year}x/year")
    print(f"Years: {years}")
    print(f"Future value: ${final:,.2f}")
    print()
    print("Year-by-year:")
    for year, balance in yearly_balances(
        principal, annual_rate, years, compounds_per_year
    ):
        print(f"  Year {year:2d}: ${balance:,.2f}")


if __name__ == "__main__":
    main()