argparse: Argparse: Add `subnamespace` flag to `_SubParsersAction.add_parser()` to allow nested `Namespace`s

[jb2170]

Feature or enhancement

Proposal:

Summary

We add the keyword-only flag subnamespace to _SubParsersAction.add_parser(), which when set to True tells the parent parser to store the subparser's parsed arguments contained in their own Namespace, nested within the parent's Namespace.

This allows hierarchical nested Namespaces to form when calling parser.parse_args(), which mirror the hierarchical nature of nested subparsers.

This also solves the problem of parsers and their subparsers having conflicting dest parameters which are subject to shadowing / conflict resolution. This has been a long standing issue for argparse #59633

By default subnamespace is False for backwards compatibility, making this feature opt-in.

Mini Example

Taken from the [implementation tests][tests]

import argparse

inet = argparse.ArgumentParser(add_help=False)
inet.add_argument("address")
inet.add_argument("port", type=int)
inet.add_argument("--use-proxy", action="store_true")

unix = argparse.ArgumentParser(add_help=False)
unix.add_argument("path")

parser = argparse.ArgumentParser(prog="my-socat")
parser.add_argument("--key-file")
action = parser.add_subparsers(required=True, dest="action")

parser_bind = action.add_parser("bind", subnamespace=True)
parser_bind.add_argument("--fork", action="store_true")
bind_family = parser_bind.add_subparsers(required=True, dest="family")

parser_bind_inet = bind_family.add_parser("inet", subnamespace=True, parents=[inet])
parser_bind_unix = bind_family.add_parser("unix", subnamespace=True, parents=[unix])

parser_connect = action.add_parser("connect", subnamespace=True)
connect_family = parser_connect.add_subparsers(required=True, dest="family")

parser_connect_inet = connect_family.add_parser("inet", subnamespace=True, parents=[inet])
parser_connect_unix = connect_family.add_parser("unix", subnamespace=True, parents=[unix])

args = parser.parse_args(["bind", "unix", "/foo/bar/socket"])

assert args == argparse.Namespace(
    key_file=None,
    action="bind",
    bind=argparse.Namespace(
        fork=False,
        family="unix",
        unix=argparse.Namespace(
            path="/foo/bar/socket"
        )
    )
)

assert args.bind.unix.path == "/foo/bar/socket"

Closes / supersedes #59633 #103639 #103640

Implementation PR incoming

Has this already been discussed elsewhere?

I have already discussed this feature proposal on Discourse

Links to previous discussion of this feature:

Full discussion thread and rationale: https://discuss.python.org/t/107202

Linked PRs

• gh-149461: argparse: Implement subnamespace feature #149462

[picnixz]

Sorry but having a DPO thread does not mean that you should open an issue. You need to first reach consensus. Issues are for actionable items, not for pending discussions (especially when it comes to argparse which we want to limit the feature expansion).

[jb2170]

I opened this issue incase one needed to get the attention of the argparse maintainer ahead of time on GitHub, if they wouldn't see the discussion thread otherwise. My bad! 😅

[picnixz]

I can tag @savannahostrowski @serhiy-storchaka so that they can participate in the DPO thread. But Serhiy is usually quite active on DPO IIRC