Basic Commands, Groups, Context¶

Commands and Groups are the building blocks for Click applications. Command wraps a function to make it into a cli command. Group wraps Commands and Groups to make them into applications. Context is how groups and commands communicate.

Commands ¶

Basic Command Example ¶

A simple command decorator takes no arguments.

@click.command()
@click.option('--count', default=1)
def hello(count):
    for x in range(count):
        click.echo("Hello!")

$ hello --count 2
Hello!
Hello!

Renaming Commands ¶

By default the command is the function name with underscores replaced by dashes. To change this pass the desired name into the first positional argument.

@click.command('say-hello')
@click.option('--count', default=1)
def hello(count):
    for x in range(count):
        click.echo("Hello!")

$ say-hello --count 2
Hello!
Hello!

Deprecating Commands ¶

To mark a command as deprecated pass in deprecated=True

@click.command('say-hello', deprecated=True)
@click.option('--count', default=1)
def hello(count):
    for x in range(count):
        click.echo("Hello!")

$ say-hello --count 2
DeprecationWarning: The command 'say-hello' is deprecated.
Hello!
Hello!

Groups ¶

Basic Group Example ¶

A group wraps one or more commands. After being wrapped, the commands are nested under that group. You can see that on the help pages and in the execution. By default, invoking the group with no command shows the help page.

@click.group()
def greeting():
    click.echo('Starting greeting ...')

@greeting.command('say-hello')
@click.option('--count', default=1)
def hello(count):
    for x in range(count):
        click.echo("Hello!")

At the top level:

$ greeting
Usage: greeting [OPTIONS] COMMAND [ARGS]...

Options:
  --help  Show this message and exit.

Commands:
  say-hello

At the command level:

$ greeting say-hello
Starting greeting ...
Hello!
$ greeting say-hello --help
Starting greeting ...
Usage: greeting say-hello [OPTIONS]

Options:
  --count INTEGER
  --help           Show this message and exit.

As you can see from the above example, the function wrapped by the group decorator executes unless it is interrupted (for example by calling the help).

Renaming Groups ¶

To have a name other than the decorated function name as the group name, pass it in as the first positional argument.

@click.group('greet-someone')
def greeting():
    click.echo('Starting greeting ...')

@greeting.command('say-hello')
@click.option('--count', default=1)
def hello(count):
    for x in range(count):
        click.echo("Hello!")

$ greet-someone say-hello
Starting greeting ...
Hello!

Group Invocation Without Command ¶

By default, if a group is passed without a command, the group is not invoked and a command automatically passes --help. To change this, pass invoke_without_command=True to the group. The context object also includes information about whether or not the group invocation would go to a command nested under it.

@click.group(invoke_without_command=True)
@click.pass_context
def cli(ctx):
    if ctx.invoked_subcommand is None:
        click.echo('I was invoked without subcommand')
    else:
        click.echo(f"I am about to invoke {ctx.invoked_subcommand}")

@cli.command()
def sync():
    click.echo('The subcommand')

$ tool
I was invoked without subcommand
$ tool sync
I am about to invoke sync
The subcommand

Group Separation ¶

Command Parameters attached to a command belong only to that command.

@click.group()
def greeting():
    pass

@greeting.command()
@click.option('--count', default=1)
def hello(count):
    for x in range(count):
        click.echo("Hello!")

@greeting.command()
@click.option('--count', default=1)
def goodbye(count):
    for x in range(count):
        click.echo("Goodbye!")

$ greeting hello --count 2
Hello!
Hello!
$ greeting goodbye --count 2
Goodbye!
Goodbye!
$ greeting
Usage: greeting [OPTIONS] COMMAND [ARGS]...

Options:
  --help  Show this message and exit.

Commands:
  goodbye
  hello

Additionally parameters for a given group belong only to that group and not to the commands under it. What this means is that options and arguments for a specific command have to be specified after the command name itself, but before any other command names.

This behavior is observable with the --help option. Suppose we have a group called tool containing a command called sub.

tool --help returns the help for the whole program (listing subcommands).
tool sub --help returns the help for the sub subcommand.
But tool --help sub treats --help as an argument for the main program. Click then invokes the callback for --help, which prints the help and aborts the program before click can process the subcommand.

Arbitrary Nesting ¶

Commands are attached to a Group. Multiple groups can be attached to another group. Groups containing multiple groups can be attached to a group, and so on. To invoke a command nested under multiple groups, all the groups under which it is nested must be invoked.

@click.group()
def cli():
    pass

# Not @click so that the group is registered now.
@cli.group()
def session():
    click.echo('Starting session')

@session.command()
def initdb():
    click.echo('Initialized the database')

@session.command()
def dropdb():
    click.echo('Dropped the database')

$ cli session initdb
Starting session
Initialized the database

Lazily Attaching Commands ¶

Most examples so far have attached the commands to a group immediately, but commands may be registered later. This could be used to split commands into multiple Python modules. Regardless of how they are attached, the commands are invoked identically.

@click.group()
def cli():
    pass

@cli.command()
def initdb():
    click.echo('Initialized the database')

@click.command()
def dropdb():
    click.echo('Dropped the database')

cli.add_command(dropdb)

$ cli initdb
Initialized the database
$ cli dropdb
Dropped the database

Context Object ¶

The Context object is how commands and groups communicate.

Auto Envvar Prefix ¶

Automatically built environment variables are 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.

@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!

Global Context Access ¶

Changelog

Added in version 5.0.

Starting with Click 5.0 it is possible to access the current context from anywhere within the same thread through the use of the get_current_context() function which returns it. This is primarily useful for accessing the context bound object as well as some flags that are stored on it to customize the runtime behavior. For instance the echo() function does this to infer the default value of the color flag.

Example usage:

def get_current_command_name():
    return click.get_current_context().info_name

It should be noted that this only works within the current thread. If you spawn additional threads then those threads will not have the ability to refer to the current context. If you want to give another thread the ability to refer to this context you need to use the context within the thread as a context manager:

def spawn_thread(ctx, func):
    def wrapper():
        with ctx:
            func()
    t = threading.Thread(target=wrapper)
    t.start()
    return t

Now the thread function can access the context like the main thread would do. However if you do use this for threading you need to be very careful as the vast majority of the context is not thread safe! You are only allowed to read from the context, but not to perform any modifications on it.

Detecting the Source of a Parameter ¶

In some situations it’s helpful to understand whether or not an option or parameter came from the command line, the environment, the default value, or Context.default_map. The Context.get_parameter_source() method can be used to find this out. It will return a member of the ParameterSource enum.

@click.command()
@click.argument('port', nargs=1, default=8080, envvar="PORT")
@click.pass_context
def cli(ctx, port):
    source = ctx.get_parameter_source("port")
    click.echo(f"Port came from {source.name}")

$ cli 8080
Port came from COMMANDLINE

$ export PORT=8080
$ cli
Port came from ENVIRONMENT

$ cli
Port came from DEFAULT

Basic Commands, Groups, Context¶

Commands ¶

Basic Command Example ¶

Renaming Commands ¶

Deprecating Commands ¶

Groups ¶

Basic Group Example ¶

Renaming Groups ¶

Group Invocation Without Command ¶

Group Separation ¶

Arbitrary Nesting ¶

Lazily Attaching Commands ¶

Context Object ¶

Auto Envvar Prefix ¶

Global Context Access ¶

Detecting the Source of a Parameter ¶

Contents

Navigation