Lesson 5: Advanced Usage — Type Safety, Flexible Input & CLI Black Magic

Most CLI tools fail not because they lack power — but because they accept garbage input.
Today you learn to prevent that. You’ll tame user chaos with:

• ✅ nargs for multiple values
• ✅ choices to limit options
• ✅ type for automatic conversion
• ✅ required and default to guide logic
• ✅ Custom actions — your CLI, your rules

nargs — Accept Multiple Values

Want an option to accept a list? Use nargs.

import argparse

parser = argparse.ArgumentParser()
parser.add_argument('--tags', nargs='+', help="Add one or more tags")
args = parser.parse_args()

print(f"Tags: {args.tags}")

Usage:

python app.py --tags urgent work personal

✅ Output:

Tags: ['urgent', 'work', 'personal']

Common Forms:

choices — Limit Accepted Input

You want the user to choose only from valid options. Use choices.

parser.add_argument('--format', choices=['json', 'csv', 'xml'], required=True)

python app.py --format yaml

💥 Output:

error: argument --format: invalid choice: 'yaml' (choose from 'json', 'csv', 'xml')

✅ Keeps your CLI clean and predictable
✅ Great for modes, output types, themes, roles, etc.

type — Convert Input Automatically

parser.add_argument('--repeat', type=int, help="Number of repetitions")

python app.py --repeat 5

✅ args.repeat is already an int
❌ If someone tries --repeat hello, it errors out

required & default

You can combine them to guide user behavior:

parser.add_argument('--user', required=True)
parser.add_argument('--role', default='member')

• --user must be passed
• --role is optional and defaults to member

Custom Actions — Write Your Own Argument Logic

Want to store input, trigger functions, or validate weird formats?
Use a custom action class.

class UppercaseAction(argparse.Action):
    def __call__(self, parser, namespace, values, option_string=None):
        setattr(namespace, self.dest, values.upper())

parser.add_argument('--shout', action=UppercaseAction)

python app.py --shout hello

✅ Output:

HELLO

Use this when:

• You need to transform input (lowercase, timestamp, slugify)
• You want to log or validate something exotic
• You want total control

Recap Table

Coming Up: Final Boss

Lesson 6: Project Build — A real CLI tool using everything we’ve learned so far
Commands, flags, groups, validation, custom logic, and export.
Your CLI won't just work — it’ll sparkle like sacred code.

Note
If you find any part of this post unclear or technically inaccurate, I would appreciate hearing from you. Improving the precision of these explanations is an ongoing process, and your feedback helps strengthen the material.