Options

Adding options to commands can be accomplished with the option() decorator. Options in Click are distinct from positional arguments.

Useful and often used kwargs are:

  • default: Passes a default.

  • help: Sets help message.

  • nargs: Sets the number of arguments.

  • required: Makes option required.

  • type: Sets Parameter Types

Option Decorator

Click expects you to pass at least two positional arguments to the option decorator. They are option name and function argument name.

@click.command()
@click.option('--string-to-echo', 'string_to_echo')
def echo(string_to_echo):
    click.echo(string_to_echo)
$ echo --help
Usage: echo [OPTIONS]

Options:
  --string-to-echo TEXT
  --help                 Show this message and exit.

However, if you don’t pass in the function argument name, then Click will try to infer it. A simple way to name your option is by taking the function argument, adding two dashes to the front and converting underscores to dashes. In this case, Click will infer the function argument name correctly so you can add only the option name.

@click.command()
@click.option('--string-to-echo')
def echo(string_to_echo):
    click.echo(string_to_echo)
$ echo --string-to-echo 'Hi!'
Hi!

More formally, Click will try to infer the function argument name by:

  1. If a positional argument name does not have a prefix, it is chosen.

  2. If a positional argument name starts with with two dashes, the first one given is chosen.

  3. The first positional argument prefixed with one dash is chosen otherwise.

The chosen positional argument is converted to lower case, up to two dashes are removed from the beginning, and other dashes are converted to underscores to get the function argument name.

Examples

Decorator Arguments

Function Name

"-f", "--foo-bar"

foo_bar

"-x"

x

"-f", "--filename", "dest"

dest

"--CamelCase"

camelcase

"-f", "-fb"

f

"--f", "--foo-bar"

f

"---f"

_f

Basic Example

A simple click.Option takes one argument. This will assume the argument is not required. If the decorated function takes an positional argument then None is passed it. This will also assume the type is str.

@click.command()
@click.option('--text')
def print_this(text):
    click.echo(text)
$ print-this --text=this
this
$ print-this
$ print-this --help
Usage: print-this [OPTIONS]

Options:
  --text TEXT
  --help       Show this message and exit.

Setting a Default

Instead of setting the type, you may set a default and Click will try to infer the type.

@click.command()
@click.option('--n', default=1)
def dots(n):
    click.echo('.' * n)
$ dots --help
Usage: dots [OPTIONS]

Options:
  --n INTEGER
  --help       Show this message and exit.

Multi Value Options

To make an option take multiple values, pass in nargs. Note only a fixed number of arguments is supported. The values are passed to the underlying function as a tuple.

@click.command()
@click.option('--pos', nargs=2, type=float)
def findme(pos):
    a, b = pos
    click.echo(f"{a} / {b}")
$ findme --pos 2.0 3.0
2.0 / 3.0

Multi Value Options as Tuples

Changelog

Added in version 4.0.

As you can see that by using nargs set to a specific number each item in the resulting tuple is of the same type. This might not be what you want. Commonly you might want to use different types for different indexes in the tuple. For this you can directly specify a tuple as type:

@click.command()
@click.option('--item', type=(str, int))
def putitem(item):
    name, id = item
    click.echo(f"name={name} id={id}")

And on the command line:

$ putitem --item peter 1338
name=peter id=1338

By using a tuple literal as type, nargs gets automatically set to the length of the tuple and the click.Tuple type is automatically used. The above example is thus equivalent to this:

@click.command()
@click.option('--item', nargs=2, type=click.Tuple([str, int]))
def putitem(item):
    name, id = item
    click.echo(f"name={name} id={id}")

Multiple Options

The multiple options format allows you to call the underlying function multiple times with one command line entry. If set, the default must be a list or tuple. Setting a string as a default will be interpreted as list of characters.

@click.command()
@click.option('--message', '-m', multiple=True)
def commit(message):
    click.echo('\n'.join(message))
$ commit -m foo -m bar -m here
foo
bar
here

Counting

To count the occurrence of an option pass in count=True. If the option is not passed in, then the count is 0. Counting is commonly used for verbosity.

@click.command()
@click.option('-v', '--verbose', count=True)
def log(verbose):
    click.echo(f"Verbosity: {verbose}")
$ log
Verbosity: 0
$ log -vvv
Verbosity: 3

Boolean

Boolean options (boolean flags) take the value True or False. The simplest case sets the default value to False if the flag is not passed, and True if it is.

import sys

@click.command()
@click.option('--shout', is_flag=True)
def info(shout):
    rv = sys.platform
    if shout:
        rv = rv.upper() + '!!!!111'
    click.echo(rv)
$ info
linux
$ info --shout
LINUX!!!!111

To implement this more explicitly, pass in on-option / off-option. Click will automatically set is_flag=True. Click always wants you to provide an enable and disable flag so that you can change the default later.

import sys

@click.command()
@click.option('--shout/--no-shout', default=False)
def info(shout):
    rv = sys.platform
    if shout:
        rv = rv.upper() + '!!!!111'
    click.echo(rv)
$ info
linux
$ info --shout
LINUX!!!!111
$ info --no-shout
linux

If a forward slash(/) is contained in your option name already, you can split the parameters using ;. In Windows / is commonly used as the prefix character.

@click.command()
@click.option('/debug;/no-debug')
def log(debug):
    click.echo(f"debug={debug}")
Changelog

Changed in version 6.0.

If you want to define an alias for the second option only, then you will need to use leading whitespace to disambiguate the format string.

import sys

@click.command()
@click.option('--shout/--no-shout', ' /-N', default=False)
def info(shout):
    rv = sys.platform
    if shout:
        rv = rv.upper() + '!!!!111'
    click.echo(rv)
$ info --help
Usage: info [OPTIONS]

Options:
  --shout / -N, --no-shout
  --help                    Show this message and exit.

Flag Value

To have an flag pass a value to the underlying function set is_flag=True and set flag_value to the value desired. This can be used to create patterns like this:

import sys

@click.command()
@click.option('--upper', 'transformation', flag_value='upper')
@click.option('--lower', 'transformation', flag_value='lower')
def info(transformation):
    click.echo(getattr(sys.platform, transformation)())
$ info --help
Usage: info [OPTIONS]

Options:
  --upper
  --lower
  --help   Show this message and exit.
$ info --upper
LINUX
$ info --lower
linux

Feature Switches

In addition to boolean flags, there are also feature switches. These are implemented by setting multiple options to the same parameter name and defining a flag value. Note that by providing the flag_value parameter, Click will implicitly set is_flag=True.

To set a default flag, assign a value of True to the flag that should be the default.

import sys

@click.command()
@click.option('--upper', 'transformation', flag_value='upper',
              default=True)
@click.option('--lower', 'transformation', flag_value='lower')
def info(transformation):
    click.echo(getattr(sys.platform, transformation)())
$ info --upper
LINUX
$ info --help
Usage: info [OPTIONS]

Options:
  --upper
  --lower
  --help   Show this message and exit.
$ info
LINUX

Prompting

In some cases, you want parameters that can be provided from the command line, but if not provided, ask for user input instead. This can be implemented with Click by defining a prompt string.

Example:

@click.command()
@click.option('--name', prompt=True)
def hello(name):
    click.echo(f"Hello {name}!")

And what it looks like:

$ hello --name=John
Hello John!
$ hello
Name: John
Hello John!

If you are not happy with the default prompt string, you can ask for a different one:

@click.command()
@click.option('--name', prompt='Your name please')
def hello(name):
    click.echo(f"Hello {name}!")

What it looks like:

$ hello
Your name please: John
Hello John!

It is advised that prompt not be used in conjunction with the multiple flag set to True. Instead, prompt in the function interactively.

By default, the user will be prompted for an input if one was not passed through the command line. To turn this behavior off, see Optional Value.

Dynamic Defaults for Prompts

The auto_envvar_prefix and default_map options for the context allow the program to read option values from the environment or a configuration file. However, this overrides the prompting mechanism, so that the user does not get the option to change the value interactively.

If you want to let the user configure the default value, but still be prompted if the option isn’t specified on the command line, you can do so by supplying a callable as the default value. For example, to get a default from the environment:

import os

@click.command()
@click.option(
    "--username", prompt=True,
    default=lambda: os.environ.get("USER", "")
)
def hello(username):
    click.echo(f"Hello, {username}!")

To describe what the default value will be, set it in show_default.

import os

@click.command()
@click.option(
    "--username", prompt=True,
    default=lambda: os.environ.get("USER", ""),
    show_default="current user"
)
def hello(username):
    click.echo(f"Hello, {username}!")
$ hello --help
Usage: hello [OPTIONS]

Options:
  --username TEXT  [default: (current user)]
  --help           Show this message and exit.

Callbacks and Eager Options

Sometimes, you want a parameter to completely change the execution flow. For instance, this is the case when you want to have a --version parameter that prints out the version and then exits the application.

Note: an actual implementation of a --version parameter that is reusable is available in Click as click.version_option(). The code here is merely an example of how to implement such a flag.

In such cases, you need two concepts: eager parameters and a callback. An eager parameter is a parameter that is handled before others, and a callback is what executes after the parameter is handled. The eagerness is necessary so that an earlier required parameter does not produce an error message. For instance, if --version was not eager and a parameter --foo was required and defined before, you would need to specify it for --version to work. For more information, see Callback Evaluation Order.

A callback is a function that is invoked with three parameters: the current Context, the current Parameter, and the value. The context provides some useful features such as quitting the application and gives access to other already processed parameters.

Here’s an example for a --version flag:

def print_version(ctx, param, value):
    if not value or ctx.resilient_parsing:
        return
    click.echo('Version 1.0')
    ctx.exit()

@click.command()
@click.option('--version', is_flag=True, callback=print_version,
              expose_value=False, is_eager=True)
def hello():
    click.echo('Hello World!')

The expose_value parameter prevents the pretty pointless version parameter from being passed to the callback. If that was not specified, a boolean would be passed to the hello script. The resilient_parsing flag is applied to the context if Click wants to parse the command line without any destructive behavior that would change the execution flow. In this case, because we would exit the program, we instead do nothing.

What it looks like:

$ hello
Hello World!
$ hello --version
Version 1.0

Values from Environment Variables

A very useful feature of Click is the ability to accept parameters from environment variables in addition to regular parameters. This allows tools to be automated much easier. For instance, you might want to pass a configuration file with a --config parameter but also support exporting a TOOL_CONFIG=hello.cfg key-value pair for a nicer development experience.

This is supported by Click in two ways. One is to automatically build environment variables which is supported for options only. To enable this feature, the auto_envvar_prefix parameter needs to be passed to the script that is invoked. Each command and parameter is then added as an uppercase underscore-separated variable. If you have a subcommand called run taking an option called reload and the prefix is WEB, then the variable is WEB_RUN_RELOAD.

Example usage:

@click.command()
@click.option('--username')
def greet(username):
    click.echo(f'Hello {username}!')

if __name__ == '__main__':
    greet(auto_envvar_prefix='GREETER')

And from the command line:

$ export GREETER_USERNAME=john
$ greet
Hello john!

When using auto_envvar_prefix with command groups, the command name needs to be included in the environment variable, between the prefix and the parameter name, i.e. PREFIX_COMMAND_VARIABLE. If you have a subcommand called run-server taking an option called host and the prefix is WEB, then the variable is WEB_RUN_SERVER_HOST.

Example:

@click.group()
@click.option('--debug/--no-debug')
def cli(debug):
    click.echo(f"Debug mode is {'on' if debug else 'off'}")

@cli.command()
@click.option('--username')
def greet(username):
    click.echo(f"Hello {username}!")

if __name__ == '__main__':
    cli(auto_envvar_prefix='GREETER')
$ export GREETER_DEBUG=false
$ export GREETER_GREET_USERNAME=John
$ cli greet
Debug mode is off
Hello John!

The second option is to manually pull values in from specific environment variables by defining the name of the environment variable on the option.

Example usage:

@click.command()
@click.option('--username', envvar='USERNAME')
def greet(username):
   click.echo(f"Hello {username}!")

if __name__ == '__main__':
    greet()

And from the command line:

$ export USERNAME=john
$ greet
Hello john!

In that case it can also be a list of different environment variables where the first one is picked.

Multiple Values from Environment Values

As options can accept multiple values, pulling in such values from environment variables (which are strings) is a bit more complex. The way Click solves this is by leaving it up to the type to customize this behavior. For both multiple and nargs with values other than 1, Click will invoke the ParamType.split_envvar_value() method to perform the splitting.

The default implementation for all types is to split on whitespace. The exceptions to this rule are the File and Path types which both split according to the operating system’s path splitting rules. On Unix systems like Linux and OS X, the splitting happens for those on every colon (:), and for Windows, on every semicolon (;).

Example usage:

@click.command()
@click.option('paths', '--path', envvar='PATHS', multiple=True,
              type=click.Path())
def perform(paths):
    for path in paths:
        click.echo(path)

if __name__ == '__main__':
    perform()

And from the command line:

$ export PATHS=./foo/bar:./test
$ perform
./foo/bar
./test

Other Prefix Characters

Click can deal with alternative prefix characters other than - for options. This is for instance useful if you want to handle slashes as parameters / or something similar. Note that this is strongly discouraged in general because Click wants developers to stay close to POSIX semantics. However in certain situations this can be useful:

@click.command()
@click.option('+w/-w')
def chmod(w):
    click.echo(f"writable={w}")

if __name__ == '__main__':
    chmod()

And from the command line:

$ chmod +w
writable=True
$ chmod -w
writable=False

Note that if you are using / as prefix character and you want to use a boolean flag you need to separate it with ; instead of /:

@click.command()
@click.option('/debug;/no-debug')
def log(debug):
    click.echo(f"debug={debug}")

if __name__ == '__main__':
    log()

Callbacks for Validation

Changelog

Changed in version 2.0.

If you want to apply custom validation logic, you can do this in the parameter callbacks. These callbacks can both modify values as well as raise errors if the validation does not work. The callback runs after type conversion. It is called for all sources, including prompts.

In Click 1.0, you can only raise the UsageError but starting with Click 2.0, you can also raise the BadParameter error, which has the added advantage that it will automatically format the error message to also contain the parameter name.

def validate_rolls(ctx, param, value):
    if isinstance(value, tuple):
        return value

    try:
        rolls, _, dice = value.partition("d")
        return int(dice), int(rolls)
    except ValueError:
        raise click.BadParameter("format must be 'NdM'")

@click.command()
@click.option(
    "--rolls", type=click.UNPROCESSED, callback=validate_rolls,
    default="1d6", prompt=True,
)
def roll(rolls):
    sides, times = rolls
    click.echo(f"Rolling a {sides}-sided dice {times} time(s)")
$ roll --rolls=42
Usage: roll [OPTIONS]
Try 'roll --help' for help.

Error: Invalid value for '--rolls': format must be 'NdM'

$ roll --rolls=2d12
Rolling a 12-sided dice 2 time(s)

$ roll
Rolls [1d6]: 42
Error: format must be 'NdM'
Rolls [1d6]: 2d12
Rolling a 12-sided dice 2 time(s)

Optional Value

Providing the value to an option can be made optional, in which case providing only the option’s flag without a value will either show a prompt or use its flag_value.

Setting is_flag=False, flag_value=value tells Click that the option can still be passed a value, but if only the flag is given the flag_value is used.

@click.command()
@click.option("--name", is_flag=False, flag_value="Flag", default="Default")
def hello(name):
    click.echo(f"Hello, {name}!")
$ hello
Hello, Default!
$ hello --name Value
Hello, Value!
$ hello --name
Hello, Flag!

If the option has prompt enabled, then setting prompt_required=False tells Click to only show the prompt if the option’s flag is given, instead of if the option is not provided at all.

@click.command()
@click.option('--name', prompt=True, prompt_required=False, default="Default")
def hello(name):
    click.echo(f"Hello {name}!")
$ hello
Hello Default!
$ hello --name Value
Hello Value!
$ hello --name
Name [Default]: 

If required=True, then the option will still prompt if it is not given, but it will also prompt if only the flag is given.