mutually_exclusive_group with optional and positio

Question

I created an cli specification with docopt which works great, however for some reason I have to rewrite it to argparse

Usage:
    update_store_products <store_name>...
    update_store_products --all

    Options:
      -a --all     Updates all stores configured in config

How to do that?

What is important I don't want to have something like this:

update_store_products [--all] <store_name>...

I think it would be rather something like this:

update_store_products (--all | <store_name>...)

I tried to use add_mutually_exclusive_group, but I got error:

ValueError: mutually exclusive arguments must be optional

Answer 1

First off, you should include the shortest code necessary to reproduce the error in the question itself. Without it an answer is just a shot in the dark.

Now, I'm willing to bet your argparse definitions look a bit something like this:

parser = ArgumentParser()
group = parser.add_mutually_exclusive_group(required=True)
group.add_argument('--all', action='store_true')
group.add_argument('store_name', nargs='*')

The arguments in a mutually exclusive group must be optional, because it would not make much sense to have a required argument there, as the group could then only have that argument ever. The nargs='*' alone is not enough – the required attribute of the created action will be True – to convince the mutex group that the argument is truly optional. What you have to do is add a default:

parser = ArgumentParser()
group = parser.add_mutually_exclusive_group(required=True)
group.add_argument('--all', action='store_true')
group.add_argument('store_name', nargs='*', default=[])

This will result in:

[~]% python2 arg.py
usage: arg.py [-h] (--all | store_name [store_name ...])
arg.py: error: one of the arguments --all store_name is required

[~]% python2 arg.py --all
Namespace(all=True, store_name=[])

[~]% python2 arg.py store1 store2 store3
Namespace(all=False, store_name=['store1', 'store2', 'store3'])