Click tools with optional arguments

Question

I want to write a CLI hello that takes a FILENAME as an argument except when the option -s STRING is given, in which case the STRING is processed directly. The program should print hello {NAME} where name is either given via the STRING or taken to be the contents of the file FILENAME.

example:

$ cat myname.txt
john
$ hello myname.txt
hello john
$ hello -s paul
hello paul

A possible workaround is to use two options:

@click.command()
@click.option("-f", "--filename", type=str)
@click.option("-s", "--string", type=str)
def main(filename: str, string: str):
 if filename:
 ...
 if string:
 ....

but that means that I must call hello with a flag.

Answer 1

You can use a Click argument and make it not required like:

import click
@click.command()
@click.argument('filename', required=False)
@click.option("-s", "--string")
def main(filename, string):
 if None not in (filename, string):
 raise click.ClickException('filename argument and option string are mutually exclusive!')
 click.echo(f'Filename: {filename}')
 click.echo(f'String: {string}')

• Wolf: Quick question for clarification: you could use argument for both options in the given case, right?
• Wolf: @self Only now I figured out that I misunderstood your suggestion. I thought argument('some') would be a shortcut for option('-s', '--some',tyle=str): of course not both can be made an optional argument! Maybe the short introduction can be too short for people new to the Click API. ... the approach shown in the code leads to a good CLI.