building-python-clis

Python CLI Development

Framework Selection

Click (Recommended): Mature, extensive features Typer: Modern, type-hint focused argparse: Zero dependencies, standard library

Click Quick Start

import click

@click.group()
@click.version_option(version='1.0.0')
def cli():
    """My CLI tool."""
    pass

@cli.command()
@click.argument('input_file', type=click.Path(exists=True))
@click.option('--output', '-o', default='-', help='Output file')
@click.option('--verbose', '-v', is_flag=True)
def process(input_file, output, verbose):
    """Process an input file."""
    if verbose:
        click.echo(f"Processing {input_file}")
    # ...

if __name__ == '__main__':
    cli()

Entry Point (pyproject.toml)

[project.scripts]
mycli = "my_package.cli:cli"

[project.optional-dependencies]
cli = ["click>=8.0"]

Common Patterns

# File I/O with stdin/stdout support
@click.argument('input', type=click.File('r'), default='-')
@click.argument('output', type=click.File('w'), default='-')

# Progress bar
with click.progressbar(items, label='Processing') as bar:
    for item in bar:
        process(item)

# Colored output
click.secho("Success!", fg='green', bold=True)
click.secho("Error!", fg='red', err=True)

# Error handling
if not valid:
    raise click.BadParameter(f'Invalid value: {value}')

Testing with CliRunner

from click.testing import CliRunner
from mypackage.cli import cli

def test_process():
    runner = CliRunner()
    result = runner.invoke(cli, ['process', 'input.txt'])
    assert result.exit_code == 0
    assert 'expected output' in result.output

def test_stdin():
    runner = CliRunner()
    result = runner.invoke(cli, ['process', '-'], input='test data\n')
    assert result.exit_code == 0

Shell Completion

# Generate completion scripts
_MYCLI_COMPLETE=bash_source mycli > ~/.mycli-complete.bash
_MYCLI_COMPLETE=zsh_source mycli > ~/.mycli-complete.zsh

For detailed patterns, see:

• CLICK_PATTERNS.md - Advanced Click usage
• TYPER_GUIDE.md - Typer alternative

CLI Checklist

Setup:
- [ ] Entry point in pyproject.toml
- [ ] --help works for all commands
- [ ] --version displays version

UX:
- [ ] Errors go to stderr with non-zero exit
- [ ] Helpful error messages
- [ ] stdin/stdout support where appropriate

Testing:
- [ ] Tests for all commands
- [ ] Test error cases
- [ ] Test stdin processing

Learn More

This skill is based on the Guide to Developing High-Quality Python Libraries by Will McGinnis. See these posts for related coverage:

• Makefiles for Python Development
• pyproject.toml Explained