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.

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. By default, the name of the parameter is the first long option defined; otherwise the first short one is used.

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

And on the command line:

$ dots --n=2
..

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

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):
    click.echo('%s / %s' % pos)

And on the command line:

$ findme --pos 2.0 3.0
2.0 / 3.0

Tuples as Multi Value Options

New 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=(unicode, int))
def putitem(item):
    click.echo('name=%s id=%d' % item)

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([unicode, int]))
def putitem(item):
    click.echo('name=%s id=%d' % item)

Multiple Options

Similarly to nargs, there is also the case of wanting to support a parameter being provided multiple times to and have all 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

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('Verbosity: %s' % 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
LINUX2!!!!111
$ info --no-shout
linux2

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
LINUX2!!!!111

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('debug=%s' % debug)

if __name__ == '__main__':
    log()

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
LINUX2
$ info --lower
linux2
$ info
LINUX2

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.

Example:

@click.command()
@click.option('--hash-type', type=click.Choice(['md5', 'sha1']))
def digest(hash_type):
    click.echo(hash_type)

What it looks like:

$ digest --hash-type=md5
md5

$ digest --hash-type=foo
Usage: digest [OPTIONS]

Error: Invalid value for "--hash-type": invalid choice: foo. (choose from md5, sha1)

$ digest --help
Usage: digest [OPTIONS]

Options:
  --hash-type [md5|sha1]
  --help                  Show this message and exit.

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('Hello %s!' % 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('Hello %s!' % name)

What it looks like:

$ hello
Your name please: John
Hello John!

Password Prompts

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

@click.command()
@click.option('--password', prompt=True, hide_input=True,
              confirmation_prompt=True)
def encrypt(password):
    click.echo('Encrypting password to %s' % password.encode('rot13'))

What it looks like:

$ encrypt
Password: 
Repeat for confirmation: 
Encrypting password to 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('Encrypting password to %s' % password.encode('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:

@click.command()
@click.option('--username', prompt=True,
              default=lambda: os.environ.get('USER', ''))
def hello(username):
    print("Hello,", username)

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 two parameters: the current Context 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(help='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 foo taking an option called bar and the prefix is MY_TOOL, then the variable is MY_TOOL_FOO_BAR.

Example usage:

@click.command()
@click.option('--username')
def greet(username):
    click.echo('Hello %s!' % username)

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

And from the command line:

$ export GREETER_USERNAME=john
$ greet
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('Hello %s!' % 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('writable=%s' % 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('debug=%s' % debug)

if __name__ == '__main__':
    log()

Range Options

A special mention should go to the IntRange type, which works very similarly to the INT type, but restricts the value to fall into a specific range (inclusive on both edges). It has two modes:

  • the default mode (non-clamping mode) where a value that falls outside of the range will cause an error.
  • an optional clamping mode where a value that falls outside of the range will be clamped. This means that a range of 0-5 would return 5 for the value 10 or 0 for the value -1 (for example).

Example:

@click.command()
@click.option('--count', type=click.IntRange(0, 20, clamp=True))
@click.option('--digit', type=click.IntRange(0, 10))
def repeat(count, digit):
    click.echo(str(digit) * count)

if __name__ == '__main__':
    repeat()

And from the command line:

$ repeat --count=1000 --digit=5
55555555555555555555
$ repeat --count=1000 --digit=12
Usage: repeat [OPTIONS]

Error: Invalid value for "--digit": 12 is not in the valid range of 0 to 10.

If you pass None for any of the edges, it means that the range is open at that side.

Callbacks for Validation

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.

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.

Example:

def validate_rolls(ctx, param, value):
    try:
        rolls, dice = map(int, value.split('d', 2))
        return (dice, rolls)
    except ValueError:
        raise click.BadParameter('rolls need to be in format NdM')

@click.command()
@click.option('--rolls', callback=validate_rolls, default='1d6')
def roll(rolls):
    click.echo('Rolling a %d-sided dice %d time(s)' % rolls)

if __name__ == '__main__':
    roll()

And what it looks like:

$ roll --rolls=42
Usage: roll [OPTIONS]

Error: Invalid value for "--rolls": rolls need to be in format NdM

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