Add One Validated Option

Positional paths answer which files to use. The remaining interface decision is optional: keep only items whose quantity reaches a caller’s threshold.

Validate before conversion starts

argparse can call a conversion function for an option value. Write one that either returns a non-negative integer or raises the parser’s own error type:

def non_negative_int(text):
    try:
        value = int(text)
    except ValueError:
        raise argparse.ArgumentTypeError("must be a non-negative integer")
    if value < 0:
        raise argparse.ArgumentTypeError("must be a non-negative integer")
    return value

This failure belongs to the interface grammar, not to catalog data. Argparse uses the ArgumentTypeError message as its explanation, shows usage on stderr, and finishes with status 2 before the input file is opened. It also catches a plain ValueError, but replaces its message with a generic invalid-value explanation. Use ArgumentTypeError when you want the caller to see your specific rule.

Add the option to build_parser:

parser.add_argument(
    "--minimum-quantity",
    type=non_negative_int,
    default=0,
    metavar="COUNT",
    help="keep items whose quantity is at least COUNT (default: 0)",
)

The default preserves the earlier converter: leaving the option out keeps every valid item.

Filter values, not files

Add select_items(items, minimum_quantity). Visit the already normalized item dictionaries, append an item when item["quantity"] >= minimum_quantity, and return the new list. Do not parse CSV again, sort, or remove duplicates.

Change convert_catalog to accept minimum_quantity=0, select after read_catalog_csv, and write the selected list through the same validated JSON writer. Finally, pass args.minimum_quantity from main.

Three things are worth checking by hand. Run with --minimum-quantity 0 and confirm you get every item, so the default really is harmless. Run with a threshold that excludes everything and confirm you get an empty item list rather than a crash. Then put some text in the output file, run with --minimum-quantity -1, and confirm your text is still there: argparse should have rejected the command before anything opened that file.