How to add common options to sub commands which can go *after* the name of the sub command

Question

Using the CLI library click I have an application script app.py with the two sub commands read and write:

@click.group()
@click.pass_context
def cli(ctx):
    pass

@cli.command()
@click.pass_context
def read(ctx):
    print("read")

@cli.command()
@click.pass_context
def write(ctx):
    print("write")

I want to declare a common option --format. I know I can add it as a option to the command group via

@click.group()
@click.option('--format', default='json')
@click.pass_context
def cli(ctx, format):
    ctx.obj['format'] = format

But then I cannot give the option after the command, which in my use case is a lot more natural. I want to be able to issue in the shell:

app.py read --format XXX 

but with the outlined set-up I get the message Error: no such option: --format. The script only accepts the option before the command.

So my question is: How can I add a common option to both sub commands so that it works as if the option were given to each sub command?

Answer 1

AFAICT, this is not possible with Click. The docs state that:

Click strictly separates parameters between commands and subcommands. What this means is that options and arguments for a specific command have to be specified after the command name itself, but before any other command names.

A possible workaround is writing a common_options decorator. The following example is using the fact that click.option is a function that returns a decorator function which expects to be applied in series. IOW, the following:

@click.option("-a")
@click.option("-b")
def hello(a, b):
    pass

is equivalent to the following:

def hello(a, b):
    pass

hello = click.option("-a")(click.option("-b")(hello))

The drawback is that you need to have the common argument set on all your subcommands. This can be resolved through **kwargs, which collects keyword arguments as a dict.

(Alternately, you could write a more advanced decorator that would feed the arguments into the context or something like that, but my simple attempt didn't work and i'm not ready to try more advanced approaches. I might edit the answer later and add them.)

With that, we can make a program:

import click
import functools

@click.group()
def cli():
    pass

def common_options(f):
    options = [
        click.option("-a", is_flag=True),
        click.option("-b", is_flag=True),
    ]
    return functools.reduce(lambda x, opt: opt(x), options, f)

@cli.command()
@common_options
def hello(**kwargs):
    print(kwargs)
    # to get the value of b:
    print(kwargs["b"])

@cli.command()
@common_options
@click.option("-c", "--citrus")
def world(citrus, a, **kwargs):
    print("citrus is", citrus)
    if a:
        print(kwargs)
    else:
        print("a was not passed")

if __name__ == "__main__":
    cli()