As well as the append action it would be convenient for there to be an extend action.

This is kind of useful to allow something like:

parser.add_argument("--foo", action="extend", nargs="+", type=str)

given

parser.parse_args("--foo f1 --foo f2 f3 f4".split())

to result in

["f1", "f2", "f3", "f4"].

The action "append" results in

[["f1"], ["f2", "f3", "f4"]]

And action store in

["f2", "f3", "f4"].

It is easy to write a custom action, but it feels like a fairly common requirement.

Probably it would make sense to extend the default, similarly to how append behaves.

My opinion is that this is an unnecessary addition.

Reasons:

- it is easy to add as a custom action class

- I haven't seen other requests for it

- the append retains information about the argument strings that extend looses

- it is easy flatten the appended attribute, e.g with a simple loop or a flattener like 'list(itertools.chain(*ll))'

> - I haven't seen other requests for it

For the record, an Extend custom action class is one of very few such classes I have ever written for argparse for exactly the OP's usecase, i.e., it is useful for any parser that should accept the same option multiple times, while accepting multiple arguments each time.
It *is* very simple to implement (and that's why I never asked for it as a feature), but only once you've understood custom action classes, which are not exactly for beginners.
All in all, I'm +1 on the request.

I've felt the need for this myself.

New changeset aa32a7e1116f7aaaef9fec453db910e90ab7b101 by Miss Islington (bot) (Batuhan Taşkaya) in branch 'master':
bpo-23378: Add an extend action to argparse (GH-13305)
https://github.com/python/cpython/commit/aa32a7e1116f7aaaef9fec453db910e90ab7b101