CLI Tools with argparse & Click

Also Known As

TL;DR

Explanation

argparse (stdlib): define arguments programmatically, automatic --help, type conversion, positional vs optional arguments. Click: decorator-based (@click.command, @click.option, @click.argument), composable subcommand groups, automatic prompting, password hiding, file handling, and better error messages. Typer: Click wrapper with type hints — Python type annotations define CLI arguments. Use argparse for simple scripts, Click/Typer for complex tools distributed as packages. Both generate --help automatically from function docstrings and argument definitions.

Common Misconception

Why It Matters

Common Mistakes

• Parsing sys.argv manually instead of using argparse/Click.
• No input validation — argparse type= parameter handles this automatically.
• Missing default values — always provide defaults or mark arguments as required explicitly.
• No subcommands for multi-action tools — Click's @click.group() organises complex CLIs cleanly.

Code Examples

# Manual sys.argv parsing — fragile:
import sys
if len(sys.argv) < 3:
    print('Usage: tool.py input output')
    sys.exit(1)
input_file = sys.argv[1]
output_file = sys.argv[2]
# No type checking, no --help, no --verbose flag support

import click

@click.group()
@click.option('--verbose', '-v', is_flag=True, help='Enable verbose output')
@click.pass_context
def cli(ctx, verbose):
    ctx.ensure_object(dict)
    ctx.obj['VERBOSE'] = verbose

@cli.command()
@click.argument('input', type=click.Path(exists=True))
@click.argument('output', type=click.Path())
@click.option('--format', type=click.Choice(['json', 'csv']), default='json')
@click.pass_context
def process(ctx, input, output, format):
    if ctx.obj['VERBOSE']: click.echo(f'Processing {input} -> {output}')
    # Auto --help generated from docstring

if __name__ == '__main__': cli()

References

• ↗ https://click.palletsprojects.com/

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