Problem with 2nd-dependent option using click

test.py

"""
Following on the answer: https://stackoverflow.com/a/46451650/671013

Fix of version https://gist.github.com/drorata/1fa68b477d94aea5d37e87aa56e09295/6678ad2056edc60ee77a4e986c626f5dcb662a2a
"""

import click 


class OptionRequiredIf(click.Option):

    def full_process_value(self, ctx, value):
        value = super(OptionRequiredIf, self).full_process_value(ctx, value)
        if value is None and ctx.params['output'] == 'file':
            msg = 'Required if --output=file'
            raise click.MissingParameter(ctx=ctx, param=self, message=msg)

        return value


@click.command()
@click.option('--output',
              type=click.Choice(['stdout', 'file']), default='stdout')
@click.option('--filename', type=click.STRING, cls=OptionRequiredIf)
def main(output, filename):
    print("output: " + output)
    if output == 'file':
        if filename is None:
            print("filename must be provided!")
        else:
            print("filename: " + str(filename))


if __name__ == "__main__":
    main()