Click

Imports

import click
from click import (
    Abort,
    BadParameter,
    ClickException,
    UsageError,
    argument,
    command,
    confirm,
    echo,
    group,
    option,
    pass_context,
    pass_obj,
    prompt,
    secho,
    style,
)
from click.testing import CliRunner
from click.shell_completion import CompletionItem, ShellComplete, add_completion_class

Core Patterns

Single command with options and arguments ✅ Current

import click


@click.command()
@click.argument("name")
@click.option("--times", "-t", type=click.INT, default=1, show_default=True)
@click.option("--loud/--quiet", default=False, help="Toggle uppercase output.")
def hello(name: str, times: int, loud: bool) -> None:
    """Greet NAME a number of TIMES."""
    msg = f"Hello, {name}!"
    if loud:
        msg = msg.upper()

    for _ in range(times):
        click.echo(msg)


if __name__ == "__main__":
    hello()

• Use @click.command() to define a CLI entry point; add inputs with @click.argument() and @click.option().
• Prefer click.echo() over print() for consistent terminal behavior; use err=True for stderr.

Command groups and subcommands ✅ Current

import click


@click.group()
def cli() -> None:
    """Top-level command group."""
    pass


@cli.command()
@click.option("--path", type=click.Path(dir_okay=False, readable=True), required=True)
def show(path: str) -> None:
    """Print a file to stdout."""
    with click.open_file(path, mode="r", encoding="utf-8") as f:
        click.echo(f.read(), nl=False)


@cli.command()
@click.argument("words", nargs=-1)
def join(words: tuple[str, ...]) -> None:
    """Join WORDS with spaces."""
    click.echo(" ".join(words))


if __name__ == "__main__":
    cli()

• Use @click.group() for multi-command CLIs; register subcommands via @group.command().
• Use click.Path(...) and click.open_file(...) for validated paths and robust file opening.

Context and object passing (pass_context, pass_obj, make_pass_decorator) ✅ Current

from __future__ import annotations

from dataclasses import dataclass

import click


@dataclass
class AppState:
    verbose: bool


pass_state = click.make_pass_decorator(AppState)


@click.group()
@click.option("--verbose/--no-verbose", default=False)
@click.pass_context
def cli(ctx: click.Context, verbose: bool) -> None:
    ctx.obj = AppState(verbose=verbose)


@cli.command()
@pass_state
def status(state: AppState) -> None:
    click.echo(f"verbose={state.verbose}")


@cli.command()
@click.pass_context
def where(ctx: click.Context) -> None:
    # Direct access to context when needed
    click.echo(f"command={ctx.command.name}")


if __name__ == "__main__":
    cli()

• Use ctx.obj to store application state; click.make_pass_decorator() provides typed access to that object.
• @click.pass_context and @click.pass_obj are for dependency injection across command layers.

Prompts, confirmation, and secure input ✅ Current

import click


@click.command()
@click.option("--username", prompt=True)
@click.password_option("--password", confirmation_prompt=True)
@click.confirmation_option("--confirm", prompt="Proceed with login?")
def login(username: str, password: str, confirm: bool) -> None:
    # Never echo passwords; Click handles masking for password options.
    if not confirm:
        raise click.Abort()
    click.echo(f"Logging in as {username} (password length={len(password)})")


if __name__ == "__main__":
    login()

• click.prompt() / prompt=True collects interactive input; click.password_option() masks input and can confirm.
• click.confirmation_option() is a reusable “are you sure?” pattern; raise click.Abort to stop cleanly.

Testing commands with CliRunner ✅ Current

import click
from click.testing import CliRunner


@click.command()
@click.option("--count", type=click.INT, default=1)
def repeat(count: int) -> None:
    for i in range(count):
        click.echo(f"line {i + 1}")


def main() -> None:
    runner = CliRunner()
    result = runner.invoke(repeat, ["--count", "3"])
    assert result.exit_code == 0
    assert "line 3" in result.output


if __name__ == "__main__":
    main()

• Use click.testing.CliRunner.invoke() to run commands without spawning subprocesses.
• Inspect Result.exit_code, Result.output, and Result.exception for assertions.

Configuration

• Defaults and display
  • Use default=... on @click.option(...).
  • Use show_default=True to show defaults in --help.
• Types and validation
  • Built-in types: click.STRING, click.INT, click.FLOAT, click.BOOL, click.UUID.
  • Structured types: click.Path, click.File, click.Choice, click.IntRange, click.FloatRange, click.DateTime, click.Tuple.
• Environment variables
  • Options can read from environment variables using @click.option(..., envvar="NAME").
• Help and version
  • click.help_option() and click.version_option() can be used to add standardized --help / --version behavior.
• Embedding vs standalone
  • Command.main(..., standalone_mode=False) prevents Click from calling sys.exit and swallowing exceptions—preferred when embedding in a larger app.

Pitfalls

Wrong: Calling a Click command like a normal function with argv

import click


@click.command()
@click.option("--count", default=1)
def cmd(count: int) -> None:
    click.echo(str(count))


cmd(["--count", "3"])  # WRONG: bypasses Click's CLI parsing

Right: Use .main() (or run under __main__) to parse argv

import click


@click.command()
@click.option("--count", default=1, type=click.INT)
def cmd(count: int) -> None:
    click.echo(str(count))


if __name__ == "__main__":
    cmd()  # parses sys.argv

# Programmatic invocation:
# cmd.main(["--count", "3"], standalone_mode=False)

Wrong: Parameter name mismatch between decorator and function signature

import click


@click.command()
@click.argument("filename")
def show(file_name: str) -> None:  # WRONG: Click expects "filename"
    click.echo(file_name)

Right: Match the Python argument name to the Click parameter name

import click


@click.command()
@click.argument("filename")
def show(filename: str) -> None:
    click.echo(filename)


if __name__ == "__main__":
    show()

Wrong: Embedding a CLI but letting Click exit the process

import click


@click.command()
def cmd() -> None:
    raise click.UsageError("bad input")


def main() -> None:
    cmd.main(["cmd"])  # WRONG for embedding: may call sys.exit

Right: Use standalone_mode=False and handle ClickException

import click


@click.command()
def cmd() -> None:
    raise click.UsageError("bad input")


def main() -> None:
    try:
        cmd.main(["cmd"], standalone_mode=False)
    except click.ClickException as e:
        # Your app decides how to report errors.
        e.show()
        raise


if __name__ == "__main__":
    main()

Wrong: Callback depending on internal “missing” sentinel behavior (8.3.x sensitive)

import click


@click.command()
@click.option("--a", callback=lambda ctx, param, value: ctx.params.get("b"))
@click.option("--b")
def cmd(a: str | None, b: str | None) -> None:
    click.echo(f"a={a!r} b={b!r}")

Right: Treat missing values as None/falsey; avoid relying on internal sentinel states

import click


@click.command()
@click.option("--b")
@click.option("--a", callback=lambda ctx, param, value: (ctx.params.get("b") or value))
def cmd(a: str | None, b: str | None) -> None:
    click.echo(f"a={a!r} b={b!r}")


if __name__ == "__main__":
    cmd()

References

• Donate
• Documentation
• Changes
• Source
• Chat

Migration from v8.1.x

• Python version support change (8.2.0) ❌ Hard Deprecation (runtime constraint)

  • Change: Click 8.2.0+ requires Python 3.10+ (3.7–3.9 dropped).
  • Migration guidance: upgrade runtime to Python 3.10+ or pin Click <8.2.0.

• click.__version__ deprecated (8.2.0, ⚠️ hard deprecation in 8.3.1) ⚠️

  • Deprecated since: 8.2.0 (hard deprecation/removal in 9.1)
  • Still works: Yes (deprecated)
  • Modern alternative:

    import importlib.metadata
    
    version = importlib.metadata.version("click")
    print(version)
    

  • Migration guidance: stop reading click.__version__; use importlib.metadata.version("click") or feature detection.

• click.BaseCommand deprecated (8.2.0, ⚠️ hard deprecation in 8.3.1) ⚠️

  • Deprecated since: 8.2.0 (will be removed in 9.0)
  • Still works: Yes (deprecated)
  • Modern alternative: subclass click.Command (or click.Group for multi-command).
  • Migration guidance: update type checks and subclassing targets to click.Command.

• click.MultiCommand deprecated (8.2.0, ⚠️ hard deprecation in 8.3.1) ⚠️

  • Deprecated since: 8.2.0 (will be removed in 9.0)
  • Still works: Yes (deprecated)
  • Modern alternative: use click.Group.
  • Migration guidance: prefer Group for custom multi-command behavior.

• Flag option handling rework (8.3.0) ✅ Current behavior change

  • Change: flag option defaults are preserved and passed as-is more consistently; special-case compatibility for default=True.
  • Migration guidance: review boolean flags and explicitly set default, flag_value, and type to match intended runtime values.

• Sentinel/UNSET propagation fixes (8.3.1) ✅ Current behavior fix

  • Change: fixes around internal sentinel values during parsing and callbacks.
  • Migration guidance: callbacks should not depend on internal missing-value sentinels; treat missing values as None/falsey and validate explicitly.

API Reference

• click.command() - Decorator to define a single command; supports help, no_args_is_help, etc.
• click.group() - Decorator to define a command group for subcommands.
• click.option() - Add an option; key params: type, default, required, multiple, envvar, callback, is_flag, flag_value.
• click.argument() - Add a positional argument; key params: nargs, type, required.
• click.echo() - Write text safely to stdout/stderr; key params: err, nl, color.
• click.secho() - echo() with styling; key params: fg, bg, bold, underline, err.
• click.style() / click.unstyle() - Apply/remove ANSI styling to strings.
• click.prompt() - Interactive prompt for input; key params: default, type, hide_input, confirmation_prompt.
• click.confirm() - Yes/no prompt; key params: default, abort.
• click.password_option() - Option decorator for masked password input; supports confirmation.
• click.version_option() - Add --version option; key params: version, prog_name, message.
• click.help_option() - Add --help option; key params: help, hidden.
• click.open_file() - Open files with Click-friendly behavior; key params: mode, encoding, errors, atomic.
• click.Path / click.File - Parameter types for paths/files with validation and automatic opening (for File).
• click.Context / click.get_current_context() - Runtime context; access params, obj, command, and manage resources via Context.with_resource.
• click.Command.main() - CLI entry runner; key params: args, prog_name, standalone_mode.
• click.testing.CliRunner.invoke() - Run a command in tests; key params: args, input, env, catch_exceptions.
• ⚠️ click.BaseCommand (deprecated; will be removed in v9.0) — use click.Command.
• ⚠️ click.MultiCommand (deprecated; will be removed in v9.0) — use click.Group.
• ⚠️ click.OptionParser (deprecated; will be removed in v9.0).
• ⚠️ click.__version__ (deprecated; will be removed in v9.1) — use importlib.metadata.version("click").

Migration

From Click v8.2.x to v8.3.1:

• Flag option default handling: In v8.3.0+, the default value for flag options (is_flag=True) is now preserved and passed through as-is to your callback/functions. For legacy code, review your usage of default and flag_value on flag options. If you relied on older transformations, update your logic and tests to expect the new behavior.
• Deprecations (hard): BaseCommand, MultiCommand, OptionParser, and __version__ are now hard deprecated and will be removed in Click 9.x. Update code to use Command, Group, and importlib.metadata.version("click") instead.
• Python compatibility: You must use Python 3.10+ for Click 8.2.0 and newer.
• Sentinel/UNSET propagation (callbacks): If you use parameter callbacks, do not rely on Click's internal missing-value sentinels. Always treat missing values as None or another explicit value.

See Click's changelog for full migration details.

Security note:
All included patterns are safe for use by AI agents within the user's project directory. No destructive, exfiltrative, or privilege-modifying actions are present or permitted.