Click Command Line Interfaces: Make options required if other optional option is unset

Question

When writing a command-line interface (CLI) with the Python click library, is it possible to define e.g. three options where the second and third one are only required if the first (optional) one was left unset?

My use case is a log-in system which allows me to authenticate either via an authentication token (option 1), or, alternatively, via username (option 2) and password (option 3).

If the token was given, there is no need to check for username and password being defined or prompting them. Otherwise, if the token was omitted then username and password become required and must be given.

Can this be done somehow using callbacks?

My code to get started which of course does not reflect the intended pattern:

@click.command()
@click.option('--authentication-token', prompt=True, required=True)
@click.option('--username', prompt=True, required=True)
@click.option('--password', hide_input=True, prompt=True, required=True)
def login(authentication_token, username, password):
    print(authentication_token, username, password)

if __name__ == '__main__':
    login()

Answer 1

This can be done by building a custom class derived from click.Option, and in that class over riding the click.Option.handle_parse_result() method like:

Custom Class:

import click

class NotRequiredIf(click.Option):
    def __init__(self, *args, **kwargs):
        self.not_required_if = kwargs.pop('not_required_if')
        assert self.not_required_if, "'not_required_if' parameter required"
        kwargs['help'] = (kwargs.get('help', '') +
            ' NOTE: This argument is mutually exclusive with %s' %
            self.not_required_if
        ).strip()
        super(NotRequiredIf, self).__init__(*args, **kwargs)

    def handle_parse_result(self, ctx, opts, args):
        we_are_present = self.name in opts
        other_present = self.not_required_if in opts

        if other_present:
            if we_are_present:
                raise click.UsageError(
                    "Illegal usage: `%s` is mutually exclusive with `%s`" % (
                        self.name, self.not_required_if))
            else:
                self.prompt = None

        return super(NotRequiredIf, self).handle_parse_result(
            ctx, opts, args)

Using Custom Class:

To use the custom class, pass the cls parameter to click.option decorator like:

@click.option('--username', prompt=True, cls=NotRequiredIf,
              not_required_if='authentication_token')

How does this work?

This works because click is a well designed OO framework. The @click.option() decorator usually instantiates a click.Option object but allows this behavior to be overridden with the cls parameter. So it is a relatively easy matter to inherit from click.Option in our own class and over ride the desired methods.

In this case we over ride click.Option.handle_parse_result() and disable the need to user/password if authentication-token token is present, and complain if both user/password are authentication-token are present.

Note: This answer was inspired by this answer

Test Code:

@click.command()
@click.option('--authentication-token')
@click.option('--username', prompt=True, cls=NotRequiredIf,
              not_required_if='authentication_token')
@click.option('--password', prompt=True, hide_input=True, cls=NotRequiredIf,
              not_required_if='authentication_token')
def login(authentication_token, username, password):
    click.echo('t:%s  u:%s  p:%s' % (
        authentication_token, username, password))

if __name__ == '__main__':
    login('--username name --password pword'.split())
    login('--help'.split())
    login(''.split())
    login('--username name'.split())
    login('--authentication-token token'.split())

Results:

from login('--username name --password pword'.split()):

t:None  u:name  p:pword

from login('--help'.split()):

Usage: test.py [OPTIONS]

Options:
  --authentication-token TEXT
  --username TEXT              NOTE: This argument is mutually exclusive with
                               authentication_token
  --password TEXT              NOTE: This argument is mutually exclusive with
                               authentication_token
  --help                       Show this message and exit.