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
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.
If there is at least one name prefixed with two dashes, the first one given is used as the name.
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 isfoo_bar"-x", the name isx"-f", "--filename", "dest", the name isdest"--CamelCase", the name iscamelcase"-f", "-fb", the name isf"--f", "--foo-bar", the name isf"---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.
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
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=(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.
