How to specify an optional and iterable flag?

[cdwilson]

I've got a task which can take an optional list of variants, where the list of variants is user-defined in the configuration:
variants = [variant_1, variant_2, variant_3, etc...]

By default (no flags), the task needs to operate with the full list of variants, but I also need a --no-variants flag to disable all variants completely (effectively --variants => variants = [variant_1, variant_2, variant_3, etc...] and --no-variants => variants = [])

In addition, I need to allow a user to specify a specific subset of the variants, e.g.
--variant=variant_1 --variant=variant_3 => variants = [variant_1, variant_3].

Right now, I'm considering using two flags --variants (boolean) and --variant (iterable), where the --variant flag would only be allowed if the --no-variants flag was not specified (i.e. variants = True). So for example, the following combinations would be valid:

• invoke task => variants = True, variant = []
• invoke task --variants => variants = True, variant = []
• invoke task --no-variants => variants = False, variant = []
• invoke task --variant=variant_1 --variant=variant_3 => variants = True, variant = [variant_1, variant_3]
• invoke task --variants --variant=variant_1 --variant=variant_3 => variants = True, variants = [variant_1, variant_3]

It feels a bit awkward to me to have two flags named --variants and --variant that I need to resolve, when I really just want behavior like the following:

• invoke task => variants = [variant_1, variant_2, variant_3, etc...]
• invoke task --variants => variants = [variant_1, variant_2, variant_3, etc...]
• invoke task --no-variants => variants = []
• invoke task --variant=variant_1 --variant=variant_3 => variants = [variant_1, variant_3]

I'm wondering if Invoke has a better way of doing this. Is there a way to specify a single flag that is both optional and iterable?