How to only require an argument if an option is true with click on python?

Question

I'm new to click, and have reached the first thing I don't understand.

I'm trying to create a @click.argument() that has the required tag to be dependent on a @click.option().

I made some code for testing this out.

@click.command()
@click.option('-t','--test',is_flag=True, default=False)
@click.argument('source', default=None, required=False)
def foo(source, test):
    if source:
        print('source is something')
        print(test)
    else:
        print('source is None')
        print(test)

if __name__ == '__main__':
    foo()

But I want you have @click.argument('source', default=None, required=False) to be required only if the option -t is given. Otherwise, it should accept not having the argument.

I tried doing things like this: @click.argument('source', default=None, required=test) but it didn't work. The variable test seems to work like a normal variable in the foo function but it doesn't seem to be defined outside of it.

I have found threads where people make custom classes. I'm very overwhelmed with this. I seem to be able to only find custom classes for options that are required based on another option; not arguments. I'm hoping for an easier solution, but if I had to use a custom class, what might it look like?

Any help would be greatly appreciated.