Python receipt CLI project: parse options and report failed input

Operation contract

The parser defines required quantity and unit-minor options with a custom positive-integer type. It parses an explicit local argument list for repeatable testing instead of depending on the test runner’s arguments. Valid input produces an integer total; invalid quantity produces ArgumentError in this teaching parser configuration.

Failure and ownership boundary

The parser uses exit_on_error=False so the local test can observe the conversion error without terminating the process. A deployed CLI must define which failures exit, how stderr is used and whether any output file was already written. Python modules: separate import-time definitions from program execution and Python pathlib files: specify encoding and close the resource owner remain part of that application boundary.

Working program

import argparse

def positive_integer(text):
    value = int(text)
    if value <= 0:
        raise argparse.ArgumentTypeError("positive integer required")
    return value

def parser():
    options = argparse.ArgumentParser(exit_on_error=False)
    options.add_argument("--quantity", type=positive_integer, required=True)
    options.add_argument("--unit-minor", type=positive_integer, required=True)
    return options

if __name__ == "__main__":
    options = parser().parse_args(["--quantity", "3", "--unit-minor", "125"])
    print(options.quantity * options.unit_minor)
    try:
        parser().parse_args(["--quantity", "-1", "--unit-minor", "125"])
    except argparse.ArgumentError:
        print("option rejected")

Output

375
option rejected

Costs and limits

This fixture parses four small tokens and performs one multiplication. It is a checked CLI boundary, not an installed package or a durable billing application; input digit limits remain a deployment choice.

Common Mistakes

• Do not parse a test runner’s arguments as application input.
• Define failure output and exit status deliberately.

Connected lessons

Python modules: separate import-time definitions from program execution, Python functions: define units, validate input and return a result, Python input exercise: accept an explicit integer grammar.

Apply this boundary

Python SQLite project: transactional batches, duplicate IDs and reopen checks, Python package layout: owned imports and module entrypoints.