hello.py

from typing import Union

Number = Union[int, float]

def multiply(a: Number, b: Number) -> Number:
    """Return the product of a and b."""
    return a * b

def main() -> None:
    """Print a greeting and demo the multiply function.

    If two numbers are provided as CLI args, multiply them; otherwise use 3 and 4.
    """
    import sys

    print("Hello, World!")

    def parse_num(s: str) -> Number:
        try:
            return int(s)
        except ValueError:
            return float(s)

    args = sys.argv[1:]
    if len(args) >= 2:
        try:
            a = parse_num(args[0])
            b = parse_num(args[1])
        except ValueError:
            print("Usage: python hello.py [a b]  # a and b must be numbers")
            return
    else:
        a, b = 3, 4

    result = multiply(a, b)
    print(f"{a} * {b} = {result}")


if __name__ == "__main__":
    main()