A real CLI tool accepts arguments and flags, validates input, prints help, and returns a meaningful exit code. Python’s built-in argparse handles all of this: positional arguments, optional --flags, defaults, types, and an auto-generated --help.

The conventions that make a tool feel professional: exit 0 on success and non-zero on failure, write errors to stderr, and guard the entry point with if __name__ == "__main__": so the file can also be imported as a module.

#!/usr/bin/env python3
import argparse, sys

def main():
    p = argparse.ArgumentParser(description="Scale a deployment")
    p.add_argument("name", help="deployment name")
    p.add_argument("-r", "--replicas", type=int, default=1)
    p.add_argument("--dry-run", action="store_true")
    args = p.parse_args()

    if args.replicas < 0:
        print("replicas must be >= 0", file=sys.stderr)
        return 1
    action = "Would scale" if args.dry_run else "Scaling"
    print(f"{action} {args.name} to {args.replicas} replicas")
    return 0

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

1. Build a CLI with one positional argument and one optional --count flag.
2. Add a --verbose boolean flag using action="store_true".
3. Make the tool return exit code 1 on invalid input and test with echo $?.
4. Run it with --help and read the auto-generated usage.

• ↗ Python — argparse tutorial