Python argparse : How can I get Namespace objects for argument groups separately?

Nothing in argparse is designed to do that.

For what it's worth, the parser starts off with two argument groups, one that displays as positionals and the other as optionals (I forget the exact titles). So in your example there will actually be 4 groups.

The parser only uses argument groups when formatting the help. For parsing all arguments are put in a master parser._actions list. And during parsing the parser only passes around one namespace object.

You could define separate parsers, with different sets of arguments, and call each with parse_known_args. That works better with optionals (flagged) arguments than with positionals. And it fragments your help.

I have explored in other SO questions a novel Namespace class that could nest values based on some sort of dotted dest (names like group1.optA, group2.optC, etc). I don't recall whether I had to customize the Action classes or not.

The basic point is that when saving a value to the namespace, a parser, or actually a Action (argument) object does:

setattr(namespace, dest, value)

That (and getattr/hasattr) is all that the parser expects of the namespace. The default Namespace class is simple, little more than a plain object subclass. But it could be more elaborate.

you can do it in this way:

import argparse
parser = argparse.ArgumentParser()

group1 = parser.add_argument_group('group1')
group1.add_argument('--test1', help="test1")

group2 = parser.add_argument_group('group2')
group2.add_argument('--test2', help="test2")

args = parser.parse_args('--test1 one --test2 two'.split())

arg_groups={}

for group in parser._action_groups:
    group_dict={a.dest:getattr(args,a.dest,None) for a in group._group_actions}
    arg_groups[group.title]=argparse.Namespace(**group_dict)

This will give you the normal args, plus dictionary arg_groups containing namespaces for each of the added groups.

(Adapted from this answer)