Options — Click Documentation (8.2.x)

# Options

Adding options to commands can be accomplished with the `option()` decorator. At runtime the decorator invokes the `Option` class. 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 type

## 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
    win32
    $ info --shout
    WIN32!!!!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
    win32
    $ info --shout
    WIN32!!!!111
    $ info --no-shout
    win32

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 `flag_value`. This automatically sets `is_flag=True`. To set a default flag, set `default=True`. Setting flag values can be used to create patterns like this:

    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 --help
    Usage: info [OPTIONS]

    Options:
      --upper
      --lower
      --help   Show this message and exit.
    $ info --upper
    WIN32
    $ info --lower
    win32
    $ info

## Values from Environment Variables

To pass in a value in from a specific environment variable use `envvar`.

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

    $ export USERNAME=john
    $ greet
    Hello john!

If a list is passed to `envvar`, the first environment variable found is picked.

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

    $ export ALT_USERNAME=Bill
    $ export USERNAME=john
    $ greet
    Hello Bill!

Variable names are:

- Case-insensitive on Windows but not on other platforms.

- Not stripped of whitespaces and should match the exact name provided to the `envvar` argument.

For flag options, there is two concepts to consider: the activation of the flag driven by the environment variable, and the value of the flag if it is activated.

The environment variable need to be interpreted, because values read from them are always strings. We need to transform these strings into boolean values that will determine if the flag is activated or not.

Here are the rules used to parse environment variable values for flag options:

- `true`, `1`, `yes`, `on`, `t`, `y` are interpreted as activating the flag

- `false`, `0`, `no`, `off`, `f`, `n` are interpreted as deactivating the flag

- The presence of the environment variable without value is interpreted as deactivating the flag

- Empty strings are interpreted as deactivating the flag

- Values are case-insensitive, so the `True`, `TRUE`, `tRuE` strings are all activating the flag

- Values are stripped of leading and trailing whitespaces before being interpreted, so the `" True "` string is transformed to `"true"` and so activates the flag

- If the flag option has a `flag_value` argument, passing that value in the environment variable will activate the flag, in addition to all the cases described above

- Any other value is interpreted as deactivating the flag

.. caution:: For boolean flags with a pair of values, the only recognized environment variable is the one provided to the `envvar` argument.

    So an option defined as `--flag\--no-flag`, with a `envvar="FLAG"` parameter, there is no magical `NO_FLAG=<anything>` variable that is recognized. Only the `FLAG=<anything>` environment variable is recognized.

Once the status of the flag has been determine to be activated or not, the `flag_value` is used as the value of the flag if it is activated. If the flag is not activated, the value of the flag is set to `None` by default.

## Multiple Options 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 on every colon (`:`), and for Windows, splitting on every semicolon (`;`).

    @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()

    $ export PATHS='./foo/bar;./test'
    $ perform
    ./foo/bar
    ./test

## Other Prefix Characters

Click can deal with prefix characters besides `-` for options. Click can use `/`, `+` as well as others. Note that alternative prefix characters are generally used very sparingly if at all within POSIX.

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

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

There are special considerations for using `/` as prefix character, see Boolean for more.

## 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 value will be `flag_value`.

    @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!