# Options

Adding options to commands can be accomplished by the `option()` decorator. Since options can come in various different versions, there are a ton of parameters to configure their behavior. Options in click are distinct from positional arguments.

## Name Your Options

Options have a name that will be used as the Python argument name when calling the decorated function. This can be inferred from the option names or given explicitly. Names are given as position arguments to the decorator.

A name is chosen in the following order

1. If a name is not prefixed, it is used as the Python argument name and not treated as an option name on the command line.

2. If there is at least one name prefixed with two dashes, the first one given is used as the name.

3. The first name prefixed with one dash is used otherwise.

To get the Python argument name, the chosen name is converted to lower case, up to two dashes are removed as the prefix, and other dashes are converted to underscores.

    @click.command()
    @click.option('-s', '--string-to-echo')
    def echo(string_to_echo):
        click.echo(string_to_echo)

    @click.command()
    @click.option('-s', '--string-to-echo', 'string')
    def echo(string):
        click.echo(string)

- `"-f", "--foo-bar"`, the name is `foo_bar`

- `"-x"`, the name is `x`

- `"-f", "--filename", "dest"`, the name is `dest`

- `"--CamelCase"`, the name is `camelcase`

- `"-f", "-fb"`, the name is `f`

- `"--f", "--foo-bar"`, the name is `f`

- `"---f"`, the name is `_f`

## Basic Value Options

The most basic option is a value option. These options accept one argument which is a value. If no type is provided, the type of the default value is used. If no default value is provided, the type is assumed to be `STRING`. Unless a name is explicitly specified, the name of the parameter is the first long option defined; otherwise the first short one is used. By default, options are not required, however to make an option required, simply pass in `required=True` as an argument to the decorator.

    @click.command()
    @click.option('--n', default=1)
    def dots(n):
        click.echo('.' * n)

    # How to make an option required
    @click.command()
    @click.option('--n', required=True, type=int)
    def dots(n):
        click.echo('.' * n)

    # How to use a Python reserved word such as `from` as a parameter
    @click.command()
    @click.option('--from', '-f', 'from_')
    @click.option('--to', '-t')
    def reserved_param_name(from_, to):
        click.echo(f"from {from_} to {to}")

And on the command line:

    $ dots --n=2
    ..

In this case the option is of type `INT` because the default value is an integer.

To show the default values when showing command help, use `show_default=True`

    @click.command()
    @click.option('--n', default=1, show_default=True)
    def dots(n):
        click.echo('.' * n)

    $ dots --help
    Usage: dots [OPTIONS]

    Options:
      --n INTEGER  [default: 1]
      --help       Show this message and exit.

For single option boolean flags, the default remains hidden if the default value is False.

    @click.command()
    @click.option('--n', default=1, show_default=True)
    @click.option("--gr", is_flag=True, show_default=True, default=False, help="Greet the world.")
    @click.option("--br", is_flag=True, show_default=True, default=True, help="Add a thematic break")
    def dots(n, gr, br):
        if gr:
            click.echo('Hello world!')
        click.echo('.' * n)
        if br:
            click.echo('-' * n)

    $ dots --help
    Usage: dots [OPTIONS]

    Options:
      --n INTEGER  [default: 1]
      --gr         Greet the world.
      --br         Add a thematic break  [default: True]
      --help       Show this message and exit.

## Multi Value Options

Sometimes, you have options that take more than one argument. For options, only a fixed number of arguments is supported. This can be configured by the `nargs` parameter. The values are then stored as a tuple.

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

And on the command line:

    $ findme --pos 2.0 3.0
    2.0 / 3.0

## Tuples as Multi Value Options

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

Similarly to `nargs`, there is also the case of wanting to support a parameter being provided multiple times and have all the values recorded – not just the last one. For instance, `git commit -m foo -m bar` would record two lines for the commit message: `foo` and `bar`. This can be accomplished with the `multiple` flag:

Example:

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

And on the command line:

    $ commit -m foo -m bar
    foo
    bar

When passing a `default` with `multiple=True`, the default value must be a list or tuple, otherwise it will be interpreted as a list of single characters.

    @click.option("--format", multiple=True, default=["json"])

## Counting

In some very rare circumstances, it is interesting to use the repetition of options to count an integer up. This can be used for verbosity flags, for instance:

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

And on the command line:

    $ log -vvv
    Verbosity: 3

## Boolean Flags

Boolean flags are options that can be enabled or disabled. This can be accomplished by defining two flags in one go separated by a slash (`/`) for enabling or disabling the option. (If a slash is in an option string, Click automatically knows that it’s a boolean flag and will pass `is_flag=True` implicitly.) Click always wants you to provide an enable and disable flag so that you can change the default later.

Example:

    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)

And on the command line:

    $ info --shout
    LINUX!!!!111
    $ info --no-shout
    linux
    $ info
    linux

If you really don’t want an off-switch, you can just define one and manually inform Click that something is a flag:

    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)

And on the command line:

    $ info --shout
    LINUX!!!!111
    $ info
    linux

Note that if a slash is contained in your option already (for instance, if you use Windows-style parameters where `/` is the prefix character), you can alternatively split the parameters through `;` instead:

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

    if __name__ == '__main__':
        log()

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:

Example:

    import sys

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

    $ info --help
    Usage: info [OPTIONS]

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

## 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)())

And on the command line:

    $ info --upper
    LINUX
    $ info --lower
    linux
    $ info
    LINUX

## Choice Options

Sometimes, you want to have a parameter be a choice of a list of values. In that case you can use `Choice` type. It can be instantiated with a list of valid values. The originally passed choice will be returned, not the str passed on the command line. Token normalization functions and `case_sensitive=False` can cause the two to be different but still match.

Example:

    @click.command()
    @click.option('--hash-type',
                  type=click.Choice(['MD5', 'SHA1'], case_sensitive=False))
    def digest(hash_type):
        click.echo(hash_type)

What it looks like:

    $ digest --hash-type=MD5
    MD5

    $ digest --hash-type=md5
    MD5

    $ digest --hash-type=foo
    Usage: digest [OPTIONS]
    Try 'digest --help' for help.

    Error: Invalid value for '--hash-type': 'foo' is not one of 'MD5', 'SHA1'.

    $ digest --help
    Usage: digest [OPTIONS]

    Options:
      --hash-type [MD5|SHA1]
      --help                  Show this message and exit.

Only pass the choices as list or tuple. Other iterables (like generators) may lead to unexpected results.

Choices work with options that have `multiple=True`. If a `default` value is given with `multiple=True`, it should be a list or tuple of valid choices.

Choices should be unique after considering the effects of `case_sensitive` and any specified token normalization function.

Changelog

Changed in version 7.1: The resulting value from an option will always be one of the originally passed choices regardless of `case_sensitive`.

## 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.

## Password Prompts

Click also supports hidden prompts and asking for confirmation. This is useful for password input:

    import codecs

    @click.command()
    @click.option(
        "--password", prompt=True, hide_input=True,
        confirmation_prompt=True
    )
    def encode(password):
        click.echo(f"encoded: {codecs.encode(password, 'rot13')}")

    $ encode
    Password:
    Repeat for confirmation:
    encoded: frperg

Because this combination of parameters is quite common, this can also be replaced with the `password_option()` decorator:

    @click.command()
    @click.password_option()
    def encrypt(password):
        click.echo(f"encoded: to {codecs.encode(password, 'rot13')}")

## 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 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

Callback Signature Changes

In Click 2.0 the signature for callbacks changed. For more information about these changes see Upgrading to 2.0.

## Yes Parameters

For dangerous operations, it’s very useful to be able to ask a user for confirmation. This can be done by adding a boolean `--yes` flag and asking for confirmation if the user did not provide it and to fail in a callback:

    def abort_if_false(ctx, param, value):
        if not value:
            ctx.abort()

    @click.command()
    @click.option('--yes', is_flag=True, callback=abort_if_false,
                  expose_value=False,
                  prompt='Are you sure you want to drop the db?')
    def dropdb():
        click.echo('Dropped all tables!')

And what it looks like on the command line:

    $ dropdb
    Are you sure you want to drop the db? [y/N]: n
    Aborted!
    $ dropdb --yes
    Dropped all tables!

Because this combination of parameters is quite common, this can also be replaced with the `confirmation_option()` decorator:

    @click.command()
    @click.confirmation_option(prompt='Are you sure you want to drop the db?')
    def dropdb():
        click.echo('Dropped all tables!')

Callback Signature Changes

In Click 2.0 the signature for callbacks changed. For more information about these changes see Upgrading to 2.0.

## 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()

## Range Options

The `IntRange` type extends the `INT` type to ensure the value is contained in the given range. The `FloatRange` type does the same for `FLOAT`.

If `min` or `max` is omitted, that side is unbounded. Any value in that direction is accepted. By default, both bounds are closed, which means the boundary value is included in the accepted range. `min_open` and `max_open` can be used to exclude that boundary from the range.

If `clamp` mode is enabled, a value that is outside the range is set to the boundary instead of failing. For example, the range `0, 5` would return `5` for the value `10`, or `0` for the value `-1`. When using `FloatRange`, `clamp` can only be enabled if both bounds are closed (the default).

    @click.command()
    @click.option("--count", type=click.IntRange(0, 20, clamp=True))
    @click.option("--digit", type=click.IntRange(0, 9))
    def repeat(count, digit):
        click.echo(str(digit) * count)

    $ repeat --count=100 --digit=5
    55555555555555555555
    $ repeat --count=6 --digit=12
    Usage: repeat [OPTIONS]
    Try 'repeat --help' for help.

    Error: Invalid value for '--digit': 12 is not in the range 0<=x<=9.

## 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.