grow_percentage.py

"""Interactively calculate percentage changes between consecutive numbers.

Usage: python3.14 grow_percentage.py
Enter one number per prompt; press Ctrl+D or Ctrl+C to exit.
"""

from __future__ import annotations


def percent_delta(basis: float, new: float) -> float:
    """Return the percentage change from *basis* to *new*."""
    if basis == 0:
        raise ValueError("the previous value must not be zero")
    return ((100 * new) / basis) - 100


def main() -> int:
    previous: float | None = None
    while True:
        try:
            value = float(input(">> "))
            if previous is not None:
                try:
                    print(f"{percent_delta(previous, value):.2f}%")
                except ValueError as exc:
                    print(f"Invalid data: {exc}")
            previous = value
        except ValueError:
            print("Invalid data: enter a number")
        except (EOFError, KeyboardInterrupt):
            print()
            return 0


if __name__ == "__main__":
    raise SystemExit(main())