.. currentmodule:: click
Adding options to commands can be accomplished by the :func:`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 :ref:`positional arguments <arguments>`.
The naming rules can be found in :ref:`parameter_names`. In short, you can refer the option implicitly by the longest dash-prefixed argument:
.. click:example::
@click.command()
@click.option('-s', '--string-to-echo')
def echo(string_to_echo):
click.echo(string_to_echo)
Or, explicitly, by giving one non-dash-prefixed argument:
.. click:example::
@click.command()
@click.option('-s', '--string-to-echo', 'string')
def echo(string):
click.echo(string)
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 :data:`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:example::
@click.command()
@click.option('--n', default=1)
def dots(n):
click.echo('.' * n)
.. click:example::
# How to make an option required
@click.command()
@click.option('--n', required=True, type=int)
def dots(n):
click.echo('.' * n)
.. click:example::
# 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('from %s to %s' % (from_, to))
And on the command line:
.. click:run:: invoke(dots, args=['--n=2'])
In this case the option is of type :data:`INT` because the default value is an integer.
To show the default values when showing command help, use show_default=True
.. click:example::
@click.command()
@click.option('--n', default=1, show_default=True)
def dots(n):
click.echo('.' * n)
.. click:run:: invoke(dots, args=['--help'])
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:example::
@click.command()
@click.option('--pos', nargs=2, type=float)
def findme(pos):
click.echo('%s / %s' % pos)
And on the command line:
.. click:run::
invoke(findme, args=['--pos', '2.0', '3.0'])
.. versionadded:: 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:example::
@click.command()
@click.option('--item', type=(str, int))
def putitem(item):
click.echo('name=%s id=%d' % item)
And on the command line:
.. click:run::
invoke(putitem, args=['--item', 'peter', '1338'])
By using a tuple literal as type, nargs gets automatically set to the length of the tuple and the :class:`click.Tuple` type is automatically used. The above example is thus equivalent to this:
.. click:example::
@click.command()
@click.option('--item', nargs=2, type=click.Tuple([str, int]))
def putitem(item):
click.echo('name=%s id=%d' % item)
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:example::
@click.command()
@click.option('--message', '-m', multiple=True)
def commit(message):
click.echo('\n'.join(message))
And on the command line:
.. click:run::
invoke(commit, args=['-m', 'foo', '-m', 'bar'])
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:example::
@click.command()
@click.option('-v', '--verbose', count=True)
def log(verbose):
click.echo('Verbosity: %s' % verbose)
And on the command line:
.. click:run::
invoke(log, args=['-vvv'])
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:
.. click: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:
.. click:run::
invoke(info, args=['--shout'])
invoke(info, args=['--no-shout'])
If you really don't want an off-switch, you can just define one and manually inform Click that something is a flag:
.. click:example::
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:
.. click:run::
invoke(info, args=['--shout'])
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:example::
@click.command()
@click.option('/debug;/no-debug')
def log(debug):
click.echo('debug=%s' % debug)
if __name__ == '__main__':
log()
.. versionchanged:: 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:
.. click: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)
.. click:run::
invoke(info, args=['--help'])
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.
.. click:example::
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:
.. click:run::
invoke(info, args=['--upper'])
invoke(info, args=['--lower'])
invoke(info)
Sometimes, you want to have a parameter be a choice of a list of values. In that case you can use :class:`Choice` type. It can be instantiated with a list of valid values.
Example:
.. click:example::
@click.command()
@click.option('--hash-type', type=click.Choice(['md5', 'sha1']))
def digest(hash_type):
click.echo(hash_type)
What it looks like:
.. click:run::
invoke(digest, args=['--hash-type=md5'])
println()
invoke(digest, args=['--hash-type=foo'])
println()
invoke(digest, args=['--help'])
Note
You should only pass the choices as list or tuple. Other iterables (like generators) may lead to surprising results.
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:example::
@click.command()
@click.option('--name', prompt=True)
def hello(name):
click.echo('Hello %s!' % name)
And what it looks like:
.. click:run::
invoke(hello, args=['--name=John'])
invoke(hello, input=['John'])
If you are not happy with the default prompt string, you can ask for a different one:
.. click:example::
@click.command()
@click.option('--name', prompt='Your name please')
def hello(name):
click.echo('Hello %s!' % name)
What it looks like:
.. click:run::
invoke(hello, input=['John'])
Click also supports hidden prompts and asking for confirmation. This is useful for password input:
.. click:example::
@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:
.. click:run::
invoke(encrypt, input=['secret', 'secret'])
Because this combination of parameters is quite common, this can also be replaced with the :func:`password_option` decorator:
.. click:example::
@click.command()
@click.password_option()
def encrypt(password):
click.echo('Encrypting password to %s' % password.encode('rot13'))
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:example::
@click.command()
@click.option('--username', prompt=True,
default=lambda: os.environ.get('USER', ''))
def hello(username):
print("Hello,", username)
To describe what the default value will be, set it in show_default.
.. click:example::
@click.command()
@click.option('--username', prompt=True,
default=lambda: os.environ.get('USER', ''),
show_default='current user')
def hello(username):
print("Hello,", username)
.. click:run:: invoke(hello, args=['--help'])
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 :func:`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
:ref:`callback-evaluation-order`.
A callback is a function that is invoked with two parameters: the current :class:`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:
.. click:example::
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:
.. click:run::
invoke(hello)
invoke(hello, args=['--version'])
Callback Signature Changes
In Click 2.0 the signature for callbacks changed. For more information about these changes see :ref:`upgrade-to-2.0`.
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:
.. click:example::
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:
.. click:run::
invoke(dropdb, input=['n'])
invoke(dropdb, args=['--yes'])
Because this combination of parameters is quite common, this can also be replaced with the :func:`confirmation_option` decorator:
.. click:example::
@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 :ref:`upgrade-to-2.0`.
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:example::
@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:
.. click:run::
invoke(greet, env={'GREETER_USERNAME': 'john'},
auto_envvar_prefix='GREETER')
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.
Example:
.. click:example::
@click.group()
@click.option('--debug/--no-debug')
def cli(debug):
click.echo('Debug mode is %s' % ('on' if debug else 'off'))
@cli.command()
@click.option('--username')
def greet(username):
click.echo('Hello %s!' % username)
if __name__ == '__main__':
cli(auto_envvar_prefix='GREETER')
.. click:run::
invoke(cli, args=['greet',],
env={'GREETER_GREET_USERNAME': 'John', 'GREETER_DEBUG': 'false'},
auto_envvar_prefix='GREETER')
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:example::
@click.command()
@click.option('--username', envvar='USERNAME')
def greet(username):
click.echo('Hello %s!' % username)
if __name__ == '__main__':
greet()
And from the command line:
.. click:run::
invoke(greet, env={'USERNAME': 'john'})
In that case it can also be a list of different environment variables where the first one is picked.
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 :meth:`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 :class:`File` and :class:`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:example::
@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:
.. click:run::
import os
invoke(perform, env={'PATHS': './foo/bar%s./test' % os.path.pathsep})
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:example::
@click.command()
@click.option('+w/-w')
def chmod(w):
click.echo('writable=%s' % w)
if __name__ == '__main__':
chmod()
And from the command line:
.. click:run::
invoke(chmod, args=['+w'])
invoke(chmod, args=['-w'])
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:example::
@click.command()
@click.option('/debug;/no-debug')
def log(debug):
click.echo('debug=%s' % debug)
if __name__ == '__main__':
log()
A special mention should go to the :class:`IntRange` type, which works very similarly to the :data:`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
0-5would return5for the value10or0for the value-1(for example).
Example:
.. click: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:
.. click:run::
invoke(repeat, args=['--count=1000', '--digit=5'])
invoke(repeat, args=['--count=1000', '--digit=12'])
If you pass None for any of the edges, it means that the range is open
at that side.
.. versionchanged:: 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 :exc:`UsageError` but starting with Click 2.0, you can also raise the :exc:`BadParameter` error, which has the added advantage that it will automatically format the error message to also contain the parameter name.
Example:
.. click: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:
.. click:run::
invoke(roll, args=['--rolls=42'])
println()
invoke(roll, args=['--rolls=2d12'])