argparse subcommands with nested namespaces

I'm not entirely sure what you're asking, but I think what you want is for an argument group or sub-command to put its arguments into a sub-namespace.

As far as I know, argparse does not do this out of the box. But it really isn't hard to do by postprocessing the result, as long as you're willing to dig under the covers a bit. (I'm guessing it's even easier to do it by subclassing ArgumentParser, but you explicitly said you don't want to do that, so I didn't try that.)

parser = argparse.ArgumentParser()
parser.add_argument('--foo')
breakfast = parser.add_argument_group('breakfast')
breakfast.add_argument('--spam')
breakfast.add_argument('--eggs')
args = parser.parse_args()

Now, the list of all destinations for breakfast options is:

[action.dest for action in breakfast._group_actions]

And the key-value pairs in args is:

args._get_kwargs()

So, all we have to to is move the ones that match. It'll be a little easier if we construct dictionaries to create the namespaces from:

breakfast_options = [action.dest for action in breakfast._group_actions]
top_names = {name: value for (name, value) in args._get_kwargs()
             if name not in breakfast_options}
breakfast_names = {name: value for (name, value) in args._get_kwargs()
                   if name in breakfast_options}
top_names['breakfast'] = argparse.Namespace(**breakfast_names)
top_namespace = argparse.Namespace(**top_names)

And that's it; top_namespace looks like:

Namespace(breakfast=Namespace(eggs=None, spam='7'), foo='bar')

Of course in this case, we've got one static group. What if you wanted a more general solution? Easy. parser._action_groups is a list of all groups, but the first two are the global positional and keyword groups. So, just iterate over parser._action_groups[2:], and do the same thing for each that you did for breakfast above.

What about sub-commands instead of groups? Similar, but the details are different. If you've kept around each subparser object, it's just whole other ArgumentParser. If not, but you did keep the subparsers object, it's a special type of Action, whose choices is a dict whose keys are the subparser names and whose values are the subparsers themselves. If you kept neither… start at parser._subparsers and figure it out from there.

At any rate, once you know how to find the names you want to move and where you want to move them, it's the same as with groups.

If you've got, in addition to global args and/or groups and subparser-specific args and/or groups, some groups that are shared by multiple subparsers… then conceptually it gets tricky, because each subparser ends up with references to the same group, and you can't move it to al of them. But fortunately, you're only dealing with exactly one subparser (or none), so you can just ignore the other subparsers and move any shared group under the selected subparser (and any group that doesn't exist in the selected subparser, either leave at the top, or throw away, or pick one subparser arbitrarily).

Nesting with Action subclasses is fine for one type of Action, but is a nuisance if you need to subclass several types (store, store true, append, etc). Here's another idea - subclass Namespace. Do the same sort of name split and setattr, but do it in the Namespace rather than the Action. Then just create an instance of the new class, and pass it to parse_args.

class Nestedspace(argparse.Namespace):
    def __setattr__(self, name, value):
        if '.' in name:
            group,name = name.split('.',1)
            ns = getattr(self, group, Nestedspace())
            setattr(ns, name, value)
            self.__dict__[group] = ns
        else:
            self.__dict__[name] = value

p = argparse.ArgumentParser()
p.add_argument('--foo')
p.add_argument('--bar', dest='test.bar')
print(p.parse_args('--foo test --bar baz'.split()))

ns = Nestedspace()
print(p.parse_args('--foo test --bar baz'.split(), ns))
p.add_argument('--deep', dest='test.doo.deep')
args = p.parse_args('--foo test --bar baz --deep doodod'.split(), Nestedspace())
print(args)
print(args.test.doo)
print(args.test.doo.deep)

producing:

Namespace(foo='test', test.bar='baz')
Nestedspace(foo='test', test=Nestedspace(bar='baz'))
Nestedspace(foo='test', test=Nestedspace(bar='baz', doo=Nestedspace(deep='doodod')))
Nestedspace(deep='doodod')
doodod

The __getattr__ for this namespace (needed for actions like count and append) could be:

def __getattr__(self, name):
    if '.' in name:
        group,name = name.split('.',1)
        try:
            ns = self.__dict__[group]
        except KeyError:
            raise AttributeError
        return getattr(ns, name)
    else:
        raise AttributeError

I've proposed several other options, but like this the best. It puts the storage details where they belong, in the Namespace, not the parser.

In this script I have modified the __call__ method of the argparse._SubParsersAction. Instead of passing the namespace on to the subparser, it passes a new one. It then adds that to the main namespace. I only change 3 lines of __call__.

import argparse

def mycall(self, parser, namespace, values, option_string=None):
    parser_name = values[0]
    arg_strings = values[1:]

    # set the parser name if requested
    if self.dest is not argparse.SUPPRESS:
        setattr(namespace, self.dest, parser_name)

    # select the parser
    try:
        parser = self._name_parser_map[parser_name]
    except KeyError:
        args = {'parser_name': parser_name,
                'choices': ', '.join(self._name_parser_map)}
        msg = _('unknown parser %(parser_name)r (choices: %(choices)s)') % args
        raise argparse.ArgumentError(self, msg)

    # CHANGES
    # parse all the remaining options into a new namespace
    # store any unrecognized options on the main namespace, so that the top
    # level parser can decide what to do with them
    newspace = argparse.Namespace()
    newspace, arg_strings = parser.parse_known_args(arg_strings, newspace)
    setattr(namespace, 'subspace', newspace) # is there a better 'dest'?

    if arg_strings:
        vars(namespace).setdefault(argparse._UNRECOGNIZED_ARGS_ATTR, [])
        getattr(namespace, argparse._UNRECOGNIZED_ARGS_ATTR).extend(arg_strings)

argparse._SubParsersAction.__call__ = mycall

# Main parser
main_parser = argparse.ArgumentParser()
main_parser.add_argument("--common")

# sub commands
subparsers = main_parser.add_subparsers(dest='command')

parser_a = subparsers.add_parser('command_a')
parser_a.add_argument("--foo")
parser_a.add_argument("--bar")

parser_b = subparsers.add_parser('command_b')
parser_b.add_argument("--biz")
parser_b.add_argument("--baz")

# parse
input = 'command_a --foo bar --bar val --filter extra'.split()
namespace = main_parser.parse_known_args(input)
print namespace

input = '--common test command_b --biz bar --baz val'.split()
namespace = main_parser.parse_args(input)
print namespace

This produces:

(Namespace(command='command_a', common=None, 
    subspace=Namespace(bar='val', foo='bar')), 
['--filter', 'extra'])

Namespace(command='command_b', common='test', 
    subspace=Namespace(baz='val', biz='bar'))

I used parse_known_args to test how extra strings are passed back to the main parser.

I dropped the parents stuff because it does not add anything to this namespace change. it is just a convenient way of defining a set of arguments that several subparsers use. argparse does not keep a record of which arguments were added via parents, and which were added directly. It is not a grouping tool

argument_groups don't help much either. They are used by the Help formatter, but not by parse_args.

I could subclass _SubParsersAction (instead of reassigning __call__), but then I'd have change the main_parse.register.

Starting from abarnert's answer, I put together the following MWE++ ;-) that handles multiple configuration groups with similar option names.

#!/usr/bin/env python2
import argparse, re

cmdl_skel = {
    'description'       : 'An example of multi-level argparse usage.',
    'opts'              : {
        '--foo' : {
            'type'    : int,
            'default' : 0,
            'help'    : 'foo help main',
        },
        '--bar' : {
            'type'    : str,
            'default' : 'quux',
            'help'    : 'bar help main',
        },
    },
    # Assume your program uses sub-programs with their options. Argparse will
    # first digest *all* defs, so opts with the same name across groups are
    # forbidden. The trick is to use the module name (=> group.title) as
    # pseudo namespace which is stripped off at group parsing
    'groups' : [
        {   'module'        : 'mod1',
            'description'   : 'mod1 description',
            'opts'          : {
                '--mod1-foo, --mod1.foo'  : {
                    'type'    : int,
                    'default' : 0,
                    'help'    : 'foo help for mod1'
                },
            },
        },
        {   'module'        : 'mod2',
            'description'   : 'mod2 description',
            'opts'          : {
                '--mod2-foo, --mod2.foo'  : {
                    'type'    : int,
                    'default' : 1,
                    'help'    : 'foo help for mod2'
                },
            },
        },
    ],
    'args'              : {
        'arg1'  : {
            'type'    : str,
            'help'    : 'arg1 help',
        },
        'arg2'  : {
            'type'    : str,
            'help'    : 'arg2 help',
        },
    }
}


def parse_args ():
    def _parse_group (parser, opt, **optd):
        # digest variants
        optv = re.split('\s*,\s*', opt)
        # this may rise exceptions...
        parser.add_argument(*optv, **optd)

    errors = {}
    parser = argparse.ArgumentParser(description=cmdl_skel['description'],
                formatter_class=argparse.ArgumentDefaultsHelpFormatter)

    # it'd be nice to loop in a single run over zipped lists, but they have
    # different lenghts...
    for opt in cmdl_skel['opts'].keys():
        _parse_group(parser, opt, **cmdl_skel['opts'][opt])

    for arg in cmdl_skel['args'].keys():
        _parse_group(parser, arg, **cmdl_skel['args'][arg])

    for grp in cmdl_skel['groups']:
        group = parser.add_argument_group(grp['module'], grp['description'])
        for mopt in grp['opts'].keys():
            _parse_group(group, mopt, **grp['opts'][mopt])

    args = parser.parse_args()

    all_group_opts = []
    all_group_names = {}
    for group in parser._action_groups[2:]:
        gtitle = group.title
        group_opts = [action.dest for action in group._group_actions]
        all_group_opts += group_opts
        group_names = {
            # remove the leading pseudo-namespace
            re.sub("^%s_" % gtitle, '', name) : value
                for (name, value) in args._get_kwargs()
                    if name in group_opts
        }
        # build group namespace
        all_group_names[gtitle] = argparse.Namespace(**group_names)

    # rebuild top namespace
    top_names = {
        name: value for (name, value) in args._get_kwargs()
            if name not in all_group_opts
    }
    top_names.update(**all_group_names)
    top_namespace = argparse.Namespace(**top_names)

    return top_namespace


def main():
    args = parse_args()

    print(str(args))
    print(args.bar)
    print(args.mod1.foo)


if __name__ == '__main__':
    main()

Then you can call it like this (mnemonic: --mod1-... are options for "mod1", etc.):

$ ./argparse_example.py one two --bar=three --mod1-foo=11231 --mod2.foo=46546
Namespace(arg1='one', arg2='two', bar='three', foo=0, mod1=Namespace(foo=11231), mod2=Namespace(foo=46546))
three
11231

Based on the answer by @abarnert, I wrote a simple function that does what the OP wants:

from argparse import Namespace, ArgumentParser


def parse_args(parser):
    assert isinstance(parser, ArgumentParser)
    args = parser.parse_args()

    # the first two argument groups are 'positional_arguments' and 'optional_arguments'
    pos_group, optional_group = parser._action_groups[0], parser._action_groups[1]
    args_dict = args._get_kwargs()
    pos_optional_arg_names = [arg.dest for arg in pos_group._group_actions] + [arg.dest for arg in optional_group._group_actions]
    pos_optional_args = {name: value for name, value in args_dict if name in pos_optional_arg_names}
    other_group_args = dict()

    # If there are additional argument groups, add them as nested namespaces
    if len(parser._action_groups) > 2:
        for group in parser._action_groups[2:]:
            group_arg_names = [arg.dest for arg in group._group_actions]
            other_group_args[group.title] = Namespace(**{name: value for name, value in args_dict if name in group_arg_names})

    # combine the positiona/optional args and the group args
    combined_args = pos_optional_args
    combined_args.update(other_group_args)
    return Namespace(**combined_args)

You just give it the ArgumentParser instance and it returns a nested NameSpace according to the group structure of the arguments.

Please check out the argpext module on PyPi, it may help you!