Optional argument in click

example.py

import click

def set_global_domain(ctx, param, value):
    if value:
        ctx.params["domain"] = "NSGlobalDomain"
        return value

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

@cli.command("set-default")
@click.option("-g", "--globalDomain", is_flag=True, callback=set_global_domain)
@click.argument("domain", required=False)
@click.argument("key")
@click.argument("value")
def set_default(domain, key, value, globaldomain=None):
    print((domain, key, value))

def main():
    cli()

if __name__ == "__main__":
    main()