Robust Python CLI script with tests

1. Explain the task with a real input and output example.
2. Say whether you can install dependencies or must stick to the standard library.
3. Save the script and tests, and run pytest before using it on real data.
4. Always try it with --dry-run first.

Script: rename-invoices · Task: rename invoice PDFs using the date and vendor from a CSV · Standard library only.

2. Code (excerpt from rename_invoices.py)

from __future__ import annotations

import argparse
import csv
import logging
import sys
from pathlib import Path

log = logging.getLogger("rename-invoices")


def new_name(date: str, vendor: str) -> str:
    """Pure function: '2025-03-14', 'Acme Paper' -> '2025-03-14_acme-paper.pdf'."""
    return f"{date}_{'-'.join(vendor.lower().split())}.pdf"


def read_csv(path: Path) -> dict[str, str]:
    with path.open(newline="", encoding="utf-8") as f:
        return {row["file"]: new_name(row["date"], row["vendor"]) for row in csv.DictReader(f)}


def main(argv: list[str] | None = None) -> int:
    p = argparse.ArgumentParser(prog="rename-invoices", description="Rename invoice PDFs from a CSV.")
    p.add_argument("folder", type=Path)
    p.add_argument("csv", type=Path)
    p.add_argument("--dry-run", action="store_true", help="show changes without applying them")
    level = p.add_mutually_exclusive_group()
    level.add_argument("-v", "--verbose", action="store_true")
    level.add_argument("-q", "--quiet", action="store_true")
    args = p.parse_args(argv)
    logging.basicConfig(level=logging.DEBUG if args.verbose else logging.ERROR if args.quiet else logging.INFO)

    try:
        changes = read_csv(args.csv)
    except (OSError, KeyError) as e:
        log.error("Cannot read CSV: %s", e)
        return 2
    for old, new in changes.items():
        src, dst = args.folder / old, args.folder / new
        if not src.exists() or dst.exists():
            log.warning("Skipped: %s", old)
            continue
        print(f"{old} -> {new}")
        if not args.dry_run:
            src.rename(dst)
    return 0


if __name__ == "__main__":
    sys.exit(main())

Exit codes: 0 success, 1 usage error (argparse uses 2 by default: [FILL IN] if you want them unified), 2 data error. The CSV date format hasn't been described: [FILL IN]…