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.

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 you may pass in any positive integer, but not -1. 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¶

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 options to take an arbitrary number of arguments (which is called variadic). The arguments are passed to the underlying function as a tuple. 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(message)
    for m in message:
        click.echo(m)

$ commit -m foo -m bar -m here
('foo', 'bar', '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.

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

Use cases for this more explicit pattern include:

• The default can be dynamic so the user can explicitly specify the option with either on or off option, or pass in no option to use the dynamic default.

• Shell scripts sometimes want to be explicit even when it’s the default

• Shell aliases can set a flag, then an invocation can add a negation of the flag

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}")

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 mark the flag as default, 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
LINUX
$ info --lower
linux
$ info
LINUX

@click.option('--upper', 'transformation', flag_value='upper', default=True)

@click.option('--upper', 'transformation', flag_value='upper', default='upper')

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

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!