cli

Python CLI Development

Overview

Python is one of the most popular languages for building command-line tools, from simple scripts to complex multi-command applications. The ecosystem ranges from the standard library's argparse to high-level frameworks like typer that generate CLIs from type hints, and rich output libraries like rich and textual for beautiful terminal interfaces.

Tool Comparison

Feature	argparse	click	typer	fire	cement
Stdlib	Yes	No	No	No	No
Approach	Imperative	Decorator-based	Type-hint-based	Introspection	Full framework
Subcommands	Yes (subparsers)	Yes (groups)	Yes (app + commands)	Automatic	Yes
Auto-completion	No (add-on)	Yes (plugin)	Yes (built-in)	No	Yes
Testing	Manual	CliRunner	CliRunner (via click)	Manual	Built-in
Rich output	Manual	Plugin ecosystem	Built-in (via rich)	No	Yes
Learning curve	Low	Medium	Low	Very low	High
Dependencies	None	click	typer, click, rich	fire	cement
Best for	Simple tools, no-dependency scripts	Medium-to-large CLIs	Modern CLIs with type safety	Quick prototypes	Enterprise CLIs

Recommendation: Use typer for new CLI projects -- it provides the best developer experience with type hints, auto-completion, and rich integration. Use argparse when you cannot add dependencies. Use click for maximum flexibility and a large plugin ecosystem.

argparse Patterns

argparse is Python's built-in argument parsing library. It requires no external dependencies.

Basic Usage

import argparse
import sys


def main() -> int:
    parser = argparse.ArgumentParser(
        prog="mytool",
        description="A tool that does useful things",
        epilog="Example: mytool process --input data.csv --format json",
    )
    parser.add_argument("input", help="Input file path")
    parser.add_argument("-o", "--output", default="-", help="Output file (default: stdout)")
    parser.add_argument("-v", "--verbose", action="store_true", help="Enable verbose output")
    parser.add_argument("--count", type=int, default=1, help="Number of repetitions")

    args = parser.parse_args()

    if args.verbose:
        print(f"Processing {args.input}...")

    return 0


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

Subcommands

import argparse


def cmd_init(args: argparse.Namespace) -> int:
    print(f"Initializing project: {args.name}")
    return 0


def cmd_build(args: argparse.Namespace) -> int:
    print(f"Building with config: {args.config}")
    return 0


def main() -> int:
    parser = argparse.ArgumentParser(prog="mytool")
    subparsers = parser.add_subparsers(dest="command", required=True)

    # init subcommand
    init_parser = subparsers.add_parser("init", help="Initialize a new project")
    init_parser.add_argument("name", help="Project name")
    init_parser.set_defaults(func=cmd_init)

    # build subcommand
    build_parser = subparsers.add_parser("build", help="Build the project")
    build_parser.add_argument("-c", "--config", default="release", help="Build config")
    build_parser.set_defaults(func=cmd_build)

    args = parser.parse_args()
    return args.func(args)

Mutually Exclusive Groups

parser = argparse.ArgumentParser()
group = parser.add_mutually_exclusive_group(required=True)
group.add_argument("--json", action="store_true", help="Output as JSON")
group.add_argument("--csv", action="store_true", help="Output as CSV")
group.add_argument("--table", action="store_true", help="Output as table")

Custom Types

import argparse
from pathlib import Path


def existing_path(value: str) -> Path:
    path = Path(value)
    if not path.exists():
        raise argparse.ArgumentTypeError(f"Path does not exist: {value}")
    return path


def port_number(value: str) -> int:
    port = int(value)
    if not (1 <= port <= 65535):
        raise argparse.ArgumentTypeError(f"Invalid port: {value} (must be 1-65535)")
    return port


parser = argparse.ArgumentParser()
parser.add_argument("--config", type=existing_path, help="Config file path")
parser.add_argument("--port", type=port_number, default=8080, help="Server port")

Click Deep Dive

Click is a mature, decorator-based framework for building CLIs. It emphasizes composability and testability.

Installation

pip install click

Basic Command

import click


@click.command()
@click.argument("name")
@click.option("--greeting", "-g", default="Hello", help="The greeting to use")
@click.option("--count", "-c", default=1, type=int, help="Number of greetings")
@click.option("--verbose", "-v", is_flag=True, help="Enable verbose output")
def greet(name: str, greeting: str, count: int, verbose: bool) -> None:
    """Greet someone NAME times."""
    if verbose:
        click.echo(f"Greeting {name} {count} time(s)...")
    for _ in range(count):
        click.echo(f"{greeting}, {name}!")


if __name__ == "__main__":
    greet()

Command Groups (Subcommands)

import click


@click.group()
@click.option("--debug/--no-debug", default=False, help="Enable debug mode")
@click.pass_context
def cli(ctx: click.Context, debug: bool) -> None:
    """My multi-command CLI tool."""
    ctx.ensure_object(dict)
    ctx.obj["debug"] = debug


@cli.command()
@click.argument("name")
@click.pass_context
def init(ctx: click.Context, name: str) -> None:
    """Initialize a new project."""
    if ctx.obj["debug"]:
        click.echo("Debug mode is on")
    click.echo(f"Initializing {name}...")


@cli.command()
@click.option("--config", "-c", default="release", help="Build configuration")
@click.pass_context
def build(ctx: click.Context, config: str) -> None:
    """Build the project."""
    click.echo(f"Building with config: {config}")


if __name__ == "__main__":
    cli()

Context and State Passing

import click


class AppConfig:
    def __init__(self, debug: bool = False, output_format: str = "text"):
        self.debug = debug
        self.output_format = output_format


pass_config = click.make_pass_decorator(AppConfig, ensure=True)


@click.group()
@click.option("--debug/--no-debug", default=False)
@click.option("--format", "output_format", type=click.Choice(["text", "json", "csv"]))
@click.pass_context
def cli(ctx: click.Context, debug: bool, output_format: str) -> None:
    ctx.obj = AppConfig(debug=debug, output_format=output_format or "text")


@cli.command()
@pass_config
def status(config: AppConfig) -> None:
    """Show status."""
    if config.output_format == "json":
        click.echo('{"status": "ok"}')
    else:
        click.echo("Status: OK")

File Handling

import click


@click.command()
@click.argument("input_file", type=click.File("r"))
@click.argument("output_file", type=click.File("w"), default="-")  # default to stdout
def process(input_file, output_file) -> None:
    """Process INPUT_FILE and write to OUTPUT_FILE."""
    for line in input_file:
        output_file.write(line.upper())


@click.command()
@click.argument("directory", type=click.Path(exists=True, file_okay=False, resolve_path=True))
def scan(directory: str) -> None:
    """Scan a DIRECTORY for files."""
    click.echo(f"Scanning: {directory}")

Testing with CliRunner

from click.testing import CliRunner
from myapp.cli import cli


def test_init_command():
    runner = CliRunner()
    result = runner.invoke(cli, ["init", "myproject"])
    assert result.exit_code == 0
    assert "Initializing myproject" in result.output


def test_build_with_debug():
    runner = CliRunner()
    result = runner.invoke(cli, ["--debug", "build", "--config", "debug"])
    assert result.exit_code == 0


def test_file_processing():
    runner = CliRunner()
    with runner.isolated_filesystem():
        with open("input.txt", "w") as f:
            f.write("hello\nworld\n")
        result = runner.invoke(process, ["input.txt", "output.txt"])
        assert result.exit_code == 0
        with open("output.txt") as f:
            assert f.read() == "HELLO\nWORLD\n"

Useful Click Decorators and Features

import click

# Password prompt (hidden input)
@click.command()
@click.option("--password", prompt=True, hide_input=True, confirmation_prompt=True)
def set_password(password: str) -> None:
    click.echo("Password set.")

# Choice type
@click.command()
@click.option("--color", type=click.Choice(["red", "green", "blue"], case_sensitive=False))
def paint(color: str) -> None:
    click.echo(f"Painting {color}")

# Range type
@click.command()
@click.option("--count", type=click.IntRange(1, 100), default=10)
def repeat(count: int) -> None:
    click.echo(f"Repeating {count} times")

# Progress bar
@click.command()
def download() -> None:
    items = range(1000)
    with click.progressbar(items, label="Downloading") as bar:
        for item in bar:
            pass  # do work

# Confirmation
@click.command()
@click.confirmation_option(prompt="Are you sure you want to delete everything?")
def delete_all() -> None:
    click.echo("Deleted everything.")

Typer Deep Dive

Typer builds on click but uses Python type hints to define CLI interfaces. It produces beautiful help text and integrates with rich out of the box.

Installation

pip install typer

# With all optional dependencies (rich, shellingham)
pip install "typer[all]"

Basic Command

import typer


def main(
    name: str,
    greeting: str = "Hello",
    count: int = 1,
    verbose: bool = False,
) -> None:
    """Greet someone by NAME."""
    if verbose:
        typer.echo(f"Greeting {name} {count} time(s)...")
    for _ in range(count):
        typer.echo(f"{greeting}, {name}!")


if __name__ == "__main__":
    typer.run(main)

Multi-Command Application

import typer
from typing import Annotated, Optional
from enum import Enum
from pathlib import Path

app = typer.Typer(help="My awesome CLI tool.")


class OutputFormat(str, Enum):
    text = "text"
    json = "json"
    csv = "csv"


@app.command()
def init(
    name: Annotated[str, typer.Argument(help="Project name")],
    template: Annotated[str, typer.Option("--template", "-t", help="Template to use")] = "default",
    force: Annotated[bool, typer.Option("--force", "-f", help="Overwrite existing")] = False,
) -> None:
    """Initialize a new project."""
    if force:
        typer.echo(f"Force-creating {name} with template {template}")
    else:
        typer.echo(f"Creating {name} with template {template}")


@app.command()
def build(
    config: Annotated[str, typer.Option(help="Build configuration")] = "release",
    output_dir: Annotated[Path, typer.Option("--output", "-o", help="Output directory")] = Path("dist"),
    format: Annotated[OutputFormat, typer.Option(help="Output format")] = OutputFormat.text,
) -> None:
    """Build the project."""
    typer.echo(f"Building [{config}] -> {output_dir} (format: {format.value})")


@app.command()
def clean(
    all: Annotated[bool, typer.Option("--all", help="Remove all artifacts")] = False,
) -> None:
    """Clean build artifacts."""
    if all:
        confirmed = typer.confirm("Remove ALL artifacts including caches?")
        if not confirmed:
            raise typer.Abort()
    typer.echo("Cleaned.")


if __name__ == "__main__":
    app()

Annotated Parameters (Recommended Pattern)

from typing import Annotated
import typer

app = typer.Typer()


@app.command()
def deploy(
    # Required argument
    environment: Annotated[str, typer.Argument(help="Target environment")],
    # Optional with short flag
    tag: Annotated[str, typer.Option("--tag", "-t", help="Image tag")] = "latest",
    # Boolean flag
    dry_run: Annotated[bool, typer.Option("--dry-run", help="Preview without deploying")] = False,
    # Prompt for input
    token: Annotated[str, typer.Option(prompt=True, hide_input=True, help="Auth token")] = ...,
    # Enum choice
    region: Annotated[
        str,
        typer.Option(help="Cloud region", click_type=typer.Choice(["us-east-1", "eu-west-1", "ap-south-1"]))
    ] = "us-east-1",
) -> None:
    """Deploy the application to a target environment."""
    if dry_run:
        typer.echo(f"[DRY RUN] Would deploy {tag} to {environment} in {region}")
    else:
        typer.echo(f"Deploying {tag} to {environment} in {region}...")

Auto-Completion

# Generate completion script
my-cli --install-completion bash
my-cli --install-completion zsh
my-cli --install-completion fish

# Show completion script without installing
my-cli --show-completion bash

Typer Testing

Typer uses Click's CliRunner under the hood:

from typer.testing import CliRunner
from myapp.cli import app

runner = CliRunner()


def test_init():
    result = runner.invoke(app, ["init", "myproject"])
    assert result.exit_code == 0
    assert "Creating myproject" in result.output


def test_init_with_force():
    result = runner.invoke(app, ["init", "myproject", "--force"])
    assert result.exit_code == 0
    assert "Force-creating" in result.output


def test_build_json_format():
    result = runner.invoke(app, ["build", "--format", "json"])
    assert result.exit_code == 0
    assert "json" in result.output


def test_clean_aborted():
    result = runner.invoke(app, ["clean", "--all"], input="n\n")
    assert result.exit_code == 1  # Aborted

Nested Command Groups (Sub-Applications)

import typer

app = typer.Typer()
users_app = typer.Typer(help="User management commands.")
app.add_typer(users_app, name="users")


@users_app.command("list")
def list_users() -> None:
    """List all users."""
    typer.echo("user1\nuser2\nuser3")


@users_app.command("create")
def create_user(name: str, email: str) -> None:
    """Create a new user."""
    typer.echo(f"Created user {name} ({email})")


# Usage: mycli users list
#        mycli users create "Jane" "jane@example.com"

Rich Library

Rich is a library for beautiful terminal output: colors, tables, progress bars, syntax highlighting, markdown rendering, and more.

Installation

pip install rich

Console Markup

from rich.console import Console

console = Console()

# Styled text
console.print("[bold red]Error:[/bold red] Something went wrong")
console.print("[green]Success![/green] Operation completed.")
console.print("[dim italic]This is dimmed and italic[/dim italic]")

# Emoji
console.print(":rocket: Launching...")

# Links
console.print("Visit [link=https://example.com]our website[/link]")

# Print with highlight (auto-detects and highlights numbers, strings, etc.)
console.print({"name": "Alice", "age": 30, "active": True})

Tables

from rich.console import Console
from rich.table import Table

console = Console()

table = Table(title="Deployment Status")
table.add_column("Service", style="cyan", no_wrap=True)
table.add_column("Version", style="magenta")
table.add_column("Status", justify="center")
table.add_column("Uptime", justify="right", style="green")

table.add_row("api-gateway", "2.3.1", "[green]Healthy[/green]", "14d 3h")
table.add_row("auth-service", "1.8.0", "[green]Healthy[/green]", "14d 3h")
table.add_row("worker", "3.0.2", "[red]Degraded[/red]", "2h 15m")
table.add_row("database", "16.1", "[green]Healthy[/green]", "30d 12h")

console.print(table)

Progress Bars

import time
from rich.progress import Progress, SpinnerColumn, TextColumn, BarColumn, TaskProgressColumn

# Simple progress bar
from rich.progress import track

for item in track(range(100), description="Processing..."):
    time.sleep(0.02)

# Advanced multi-task progress
with Progress(
    SpinnerColumn(),
    TextColumn("[progress.description]{task.description}"),
    BarColumn(),
    TaskProgressColumn(),
) as progress:
    download_task = progress.add_task("Downloading...", total=1000)
    install_task = progress.add_task("Installing...", total=500)

    while not progress.finished:
        progress.update(download_task, advance=5)
        progress.update(install_task, advance=2)
        time.sleep(0.01)

Panels, Trees, and Columns

from rich.console import Console
from rich.panel import Panel
from rich.tree import Tree
from rich.columns import Columns

console = Console()

# Panel
console.print(Panel("This is important information", title="Notice", border_style="yellow"))

# Tree
tree = Tree("[bold]Project Structure[/bold]")
src = tree.add("[folder]src/")
src.add("[file]main.py")
src.add("[file]config.py")
utils = src.add("[folder]utils/")
utils.add("[file]helpers.py")
tests = tree.add("[folder]tests/")
tests.add("[file]test_main.py")
console.print(tree)

# Columns
data = [Panel(f"Item {i}", expand=True) for i in range(12)]
console.print(Columns(data))

Live Display

import time
from rich.live import Live
from rich.table import Table


def generate_table(step: int) -> Table:
    table = Table()
    table.add_column("Step")
    table.add_column("Status")
    for i in range(step + 1):
        table.add_row(f"Step {i}", "[green]Complete[/green]")
    if step < 5:
        table.add_row(f"Step {step + 1}", "[yellow]Running...[/yellow]")
    return table


with Live(generate_table(0), refresh_per_second=4) as live:
    for step in range(6):
        time.sleep(0.5)
        live.update(generate_table(step))

Syntax Highlighting and Logging

from rich.console import Console
from rich.syntax import Syntax
import logging
from rich.logging import RichHandler

console = Console()

# Syntax highlighting
code = '''
def fibonacci(n: int) -> int:
    if n <= 1:
        return n
    return fibonacci(n - 1) + fibonacci(n - 2)
'''
syntax = Syntax(code, "python", theme="monokai", line_numbers=True)
console.print(syntax)

# Rich logging handler
logging.basicConfig(
    level=logging.DEBUG,
    format="%(message)s",
    handlers=[RichHandler(rich_tracebacks=True)],
)
log = logging.getLogger("myapp")
log.info("Application started")
log.warning("Low disk space")
log.error("Connection failed")

Textual for TUI Applications

Textual is a framework for building terminal user interfaces (TUIs) with a CSS-like styling system and reactive widget model.

Installation

pip install textual

# With dev tools (for live CSS editing)
pip install "textual[dev]"

Basic Application

from textual.app import App, ComposeResult
from textual.containers import Horizontal, Vertical
from textual.widgets import Header, Footer, Static, Button, Input, DataTable


class MyApp(App):
    """A simple Textual application."""

    CSS = """
    Screen {
        layout: vertical;
    }
    #sidebar {
        width: 30;
        background: $surface;
        padding: 1;
    }
    #main {
        width: 1fr;
        padding: 1;
    }
    Button {
        margin: 1 0;
    }
    """

    BINDINGS = [
        ("q", "quit", "Quit"),
        ("d", "toggle_dark", "Toggle Dark Mode"),
    ]

    def compose(self) -> ComposeResult:
        yield Header()
        with Horizontal():
            with Vertical(id="sidebar"):
                yield Static("Navigation", classes="title")
                yield Button("Dashboard", id="btn-dashboard", variant="primary")
                yield Button("Settings", id="btn-settings")
                yield Button("Logs", id="btn-logs")
            with Vertical(id="main"):
                yield Static("Welcome to MyApp", id="content")
        yield Footer()

    def on_button_pressed(self, event: Button.Pressed) -> None:
        content = self.query_one("#content", Static)
        content.update(f"You clicked: {event.button.id}")

    def action_toggle_dark(self) -> None:
        self.dark = not self.dark


if __name__ == "__main__":
    MyApp().run()

Widgets

from textual.app import App, ComposeResult
from textual.widgets import DataTable, Input, ListView, ListItem, Label, RichLog


class DataApp(App):
    def compose(self) -> ComposeResult:
        yield Input(placeholder="Search...", id="search")
        yield DataTable()
        yield RichLog(id="log")

    def on_mount(self) -> None:
        table = self.query_one(DataTable)
        table.add_columns("Name", "Role", "Status")
        table.add_rows([
            ("Alice", "Engineer", "Active"),
            ("Bob", "Designer", "Away"),
            ("Carol", "Manager", "Active"),
        ])

    def on_input_changed(self, event: Input.Changed) -> None:
        log = self.query_one("#log", RichLog)
        log.write(f"Search: {event.value}")

Reactive Attributes

from textual.app import App, ComposeResult
from textual.reactive import reactive
from textual.widgets import Static


class Counter(Static):
    count: reactive[int] = reactive(0)

    def render(self) -> str:
        return f"Count: {self.count}"

    def watch_count(self, new_value: int) -> None:
        if new_value > 10:
            self.styles.color = "red"

    def on_click(self) -> None:
        self.count += 1

CSS Styling

Textual uses a CSS-like language for styling. Styles can be defined inline, in the CSS class variable, or in external .tcss files.

class MyApp(App):
    CSS_PATH = "styles.tcss"  # Load from external file

/* styles.tcss */
Screen {
    layout: horizontal;
}

#sidebar {
    width: 25%;
    background: $surface;
    border-right: solid $primary;
    padding: 1 2;
}

#main {
    width: 75%;
    padding: 1 2;
}

Button {
    width: 100%;
    margin: 1 0;
}

Button:hover {
    background: $primary-lighten-2;
}

DataTable > .datatable--header {
    background: $primary;
    color: $text;
}

Output Formatting

JSON Output Mode

Provide a --json flag for machine-readable output:

import json
import typer
from typing import Annotated

app = typer.Typer()


@app.command()
def status(
    json_output: Annotated[bool, typer.Option("--json", help="Output as JSON")] = False,
) -> None:
    """Show system status."""
    data = {
        "status": "healthy",
        "services": [
            {"name": "api", "status": "up", "latency_ms": 42},
            {"name": "db", "status": "up", "latency_ms": 5},
        ],
        "version": "1.2.0",
    }

    if json_output:
        typer.echo(json.dumps(data, indent=2))
    else:
        from rich.console import Console
        from rich.table import Table

        console = Console()
        console.print(f"[bold]Status:[/bold] {data['status']}")
        table = Table()
        table.add_column("Service")
        table.add_column("Status")
        table.add_column("Latency")
        for svc in data["services"]:
            table.add_row(svc["name"], svc["status"], f"{svc['latency_ms']}ms")
        console.print(table)

Color/No-Color Support

Respect the NO_COLOR environment variable (see https://no-color.org):

import os
from rich.console import Console

# Rich automatically respects NO_COLOR
console = Console(no_color=os.environ.get("NO_COLOR") is not None)

# Or force no color via CLI flag
import typer

app = typer.Typer()

@app.callback()
def main(no_color: bool = False) -> None:
    if no_color:
        os.environ["NO_COLOR"] = "1"

Table Formatting with tabulate

from tabulate import tabulate

data = [
    ["api-gateway", "2.3.1", "Healthy"],
    ["auth-service", "1.8.0", "Healthy"],
    ["worker", "3.0.2", "Degraded"],
]
headers = ["Service", "Version", "Status"]

# Multiple output formats
print(tabulate(data, headers=headers, tablefmt="grid"))
print(tabulate(data, headers=headers, tablefmt="github"))
print(tabulate(data, headers=headers, tablefmt="plain"))

Exit Codes and Error Handling

Standard Exit Codes

import sys

EXIT_SUCCESS = 0
EXIT_GENERAL_ERROR = 1
EXIT_USAGE_ERROR = 2        # Invalid arguments
EXIT_NOT_FOUND = 3          # Resource not found
EXIT_PERMISSION_DENIED = 4  # Insufficient permissions


def main() -> int:
    try:
        result = do_work()
        return EXIT_SUCCESS
    except FileNotFoundError as e:
        print(f"Error: {e}", file=sys.stderr)
        return EXIT_NOT_FOUND
    except PermissionError as e:
        print(f"Error: {e}", file=sys.stderr)
        return EXIT_PERMISSION_DENIED
    except Exception as e:
        print(f"Error: {e}", file=sys.stderr)
        return EXIT_GENERAL_ERROR


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

Error Handling in Typer

import typer
from rich.console import Console

app = typer.Typer()
err_console = Console(stderr=True)


@app.command()
def deploy(environment: str) -> None:
    """Deploy to an environment."""
    valid_envs = {"dev", "staging", "production"}
    if environment not in valid_envs:
        err_console.print(f"[red]Error:[/red] Unknown environment '{environment}'")
        err_console.print(f"Valid environments: {', '.join(sorted(valid_envs))}")
        raise typer.Exit(code=2)

    if environment == "production":
        confirmed = typer.confirm("Deploy to PRODUCTION?", abort=True)

    typer.echo(f"Deploying to {environment}...")

Error Handling in Click

import click
import sys


class AppError(click.ClickException):
    """Custom error with colored output."""

    def format_message(self) -> str:
        return f"Error: {self.message}"


@click.command()
@click.argument("config_path", type=click.Path(exists=True))
def run(config_path: str) -> None:
    try:
        load_config(config_path)
    except ValueError as e:
        raise AppError(str(e))
    except Exception as e:
        click.echo(f"Fatal: {e}", err=True)
        sys.exit(1)

CLI Testing Strategies

Testing with pytest and CliRunner

import pytest
from typer.testing import CliRunner
from myapp.cli import app

runner = CliRunner()


class TestInitCommand:
    def test_basic_init(self):
        result = runner.invoke(app, ["init", "myproject"])
        assert result.exit_code == 0
        assert "myproject" in result.output

    def test_init_with_template(self):
        result = runner.invoke(app, ["init", "myproject", "--template", "fastapi"])
        assert result.exit_code == 0
        assert "fastapi" in result.output

    def test_init_invalid_name(self):
        result = runner.invoke(app, ["init", ""])
        assert result.exit_code != 0

    def test_help_text(self):
        result = runner.invoke(app, ["init", "--help"])
        assert result.exit_code == 0
        assert "Initialize" in result.output


class TestOutputFormats:
    def test_json_output(self):
        result = runner.invoke(app, ["status", "--json"])
        assert result.exit_code == 0
        import json
        data = json.loads(result.output)
        assert "status" in data

    def test_text_output(self):
        result = runner.invoke(app, ["status"])
        assert result.exit_code == 0
        assert "Status:" in result.output

Testing with Filesystem Isolation

from click.testing import CliRunner
from myapp.cli import app


def test_file_creation():
    runner = CliRunner()
    with runner.isolated_filesystem():
        result = runner.invoke(app, ["init", "testproject"])
        assert result.exit_code == 0

        # Verify files were created
        from pathlib import Path
        assert Path("testproject/pyproject.toml").exists()
        assert Path("testproject/src").is_dir()

Testing with Environment Variables

def test_respects_no_color():
    runner = CliRunner()
    result = runner.invoke(app, ["status"], env={"NO_COLOR": "1"})
    assert result.exit_code == 0
    # Verify no ANSI escape codes in output
    assert "\033[" not in result.output

Snapshot Testing

import pytest
from syrupy.assertion import SnapshotAssertion
from typer.testing import CliRunner
from myapp.cli import app

runner = CliRunner()


def test_help_output(snapshot: SnapshotAssertion):
    result = runner.invoke(app, ["--help"])
    assert result.output == snapshot

Best Practices

1. Write --help for everything. Every command and option should have a clear help string. Users will read it before your docs.

2. Support --json output. Machine-readable output makes your CLI composable with other tools (jq, scripts, CI pipelines).

3. Write to stderr for diagnostics. Use stderr for progress, warnings, and errors. Reserve stdout for data output:

   import sys
   print("data output")              # stdout -- pipeable
   print("Warning: ...", file=sys.stderr)  # stderr -- visible but not piped
   

4. Use exit codes consistently. Return 0 for success, non-zero for errors. Document your exit codes.

5. Respect NO_COLOR. Check the NO_COLOR environment variable and disable colors/formatting when set.

6. Test with CliRunner. Both Click and Typer provide CliRunner for testing without spawning subprocesses.

7. Add shell completion. Typer generates it for free. For Click, use click-completion or click's built-in support.

8. Use Annotated parameters in Typer (not positional defaults) for clearer, more maintainable code.

9. Provide --verbose and --quiet flags. Let users control output verbosity. Consider using Python's logging module with configurable levels.

10. Version flag. Always provide --version:

    from importlib.metadata import version
    
    app = typer.Typer()
    
    def version_callback(value: bool) -> None:
        if value:
            typer.echo(f"myapp {version('my-package')}")
            raise typer.Exit()
    
    @app.callback()
    def main(
        version: Annotated[
            bool, typer.Option("--version", callback=version_callback, is_eager=True)
        ] = False,
    ) -> None:
        pass