Adding options to CLI commands

In the previous post, we used setuptools to package our app and enable us to run it using a single executable command hello. In this post, we look at how to make CLI apps more functional and interactive using options.

Options

Options are used to alter the default behavior of commands. This is often accomplished by passing in parameters in the form of --option or -o for short.

Let's assume that instead of just printing out "Hello World!", we wanted to have the option of printing it in all lowercase characters or all uppercase characters. We can add this functionality by adding an option() decorator.

# helloworld.py

import click

@click.command()
@click.option('-c', '--case', type=click.Choice(['upper', 'lower']))
def hello(case):
    if case:
        click.echo('You selected {} case.'.format(case))
    # click.echo('Hello World!')

Every time a change is made to the code, we'll have to install the app again and run it to see the changes.
Edit: I was wrong about this. The only time you have to reinstall is when you make changes to setup.py. Thanks to Fernando for the heads-up.

Then run the app, passing in the created option:

(venv) $ hello -c upper  # --case would also work
You selected upper case.

(venv) $ hello -c lower
You selected lower case.

Now try passing in some other option, like "normal":

(venv) $ hello -c normal
Usage: hello [OPTIONS]
Try "hello --help" for help.

Error: Invalid value for "-c" / "--case": invalid choice: normal. (choose from upper, lower)

It gives us an error: invalid choice: normal and even guides us on the options to choose from (choose from upper, lower), since that's what we specified type=click.Choice(['upper', 'lower']). Error handling on that part is handled by default.

Ok, now that the option is passed in successfully, we can add in the logic to see our desired output.

import click

@click.command()
@click.option('-c', '--case', type=click.Choice(['upper', 'lower']))
def hello(case):
    response = "Hello World!"
    if case == 'upper':
        click.echo(response.upper())
    elif case == 'lower':
        click.echo(response.lower())
    else:
        click.echo(response)

All we've done is add an if-else to check the option passed in, and return the text in either upper case, lower case, or just normal. Now when we install our app again and run it, we get these results:

(venv) $ hello
Hello World!

(venv) $ hello -c upper
HELLO WORLD!

(venv) $ hello -c lower
hello world!

In the next post, we'll enhance our command even more by adding arguments.