I tried to add arguments to process DB-related settings, either from typing import Optional
from a file
or explicitly specifying them. In theory with nested groups (by add_argument_group
and add_mutually_exlusive_group) this can be implemented in almost straightforward way:

# test.py
import argparse

parser = argparse.ArgumentParser()
dbsettings = parser.add_argument_group('Database settings')
xdbgrp = dbsettings.add_mutually_exclusive_group(required=True)
xdbgrp.add_argument('--db-config')

grp = xdbgrp.add_argument_group(required=True)
grp.add_argument('--db-host')
grp.add_argument('--db-user')

xgrp = grp.add_mutually_exclusive_group()
xgrp.add_argument('--db-password')
xgrp.add_argument('--db-password-file')
parser.parse_args()

But there are issues:

1. the add_mutually_exclusive_group has only one optional parameter, required=False by default,
   so I cannot provide a title, I have to create yet another group (xdbgrp in the example)

2. I would expect the parser do the complete argument parsing and validation, so I don't
   need to implement certain steps. In this example I expect to have a --db-host arg
   if the --db-config is not specified. But if I add required=True, the argparse
   module expects that with --db-config the --db-host is also specified.
   In other words the xdbgrp mutually exclusive group fails to be mutually exclusive.

3. While xgrp behaves correctly, I cannot specify both --db-password and --db-password-file,
   I still can specify them with --db-config (see Rename README to README.rst and enhance formatting #2)

4. If I run it as: python3 test.py --db-host x
   the command fails:
   usage: test.py [-h] --db-config DB_CONFIG --db-host DB_HOST [--db-user DB_USER]
   [--db-password DB_PASSWORD | --db-password-file DB_PASSWORD_FILE]
   test.py: error: one of the arguments --db-config is required

   So both --db-config and --db-host are required, the embedded group, grp fails to work,
   or prehaps again the xdbgrp fails (depends on the view)

5. Removing all required=True options the following is accepted:
   python3 test.py --db-host x --db-config y
   so the xdbgrp mutually exclusive group again doesn't work.

6. Only xdbgrp is required, --db-host is not:
   python3 test.py --db-host x
   usage: test.py [-h] --db-config DB_CONFIG [--db-host DB_HOST] [--db-user DB_USER]
   [--db-password DB_PASSWORD | --db-password-file DB_PASSWORD_FILE]
   test.py: error: one of the arguments --db-config is required
   Again, the group embedded into a mutually exclusive group is not handled correctly

What is expected:

1. add_mutually_exclusive_group can have title/description, but unfortunately it is not
   backward compatible

2. If I add a mutally exclusive group, it has XOR relation between its arguments and
   argument groups.

3. An argument group is handled as a single entity similar to an argument.
   Basically this is the same as Rename README to README.rst and enhance formatting #2.

4. A required argument affects only its argument group and the parent group
   and so on till the parser, but this chain stops at a mutually exclusive group,
   based on Rename README to README.rst and enhance formatting #2 and bpo-29403: Fix mock's broken autospec behavior on method-bound builtin functions #3 .

Checking the code the add_mutually_exclusive_group has only kwargs, so one part can be fixed (POC, breaks tests):

--- Lib/argparse.py
+++ Lib/argparse.py
@@ -1648,8 +1648,8 @@ def _remove_action(self, action):

 class _MutuallyExclusiveGroup(_ArgumentGroup):

-    def __init__(self, container, required=False):
-        super(_MutuallyExclusiveGroup, self).__init__(container)
+    def __init__(self, container, required=False, title=None, description=None):
+        super(_MutuallyExclusiveGroup, self).__init__(container, title, description)
         self.required = required
         self._container = container

@@ -2529,6 +2529,14 @@ def format_help(self):
formatter.add_arguments(action_group._group_actions)
formatter.end_section()

+ for mutual_group in self._mutually_exclusive_groups:
+ if not mutual_group.title:
+ continue
+ formatter.start_section(mutual_group.title)
+ formatter.add_text(mutual_group.description)
+ formatter.add_arguments(mutual_group._group_actions)
+ formatter.end_section()
+
# epilog
formatter.add_text(self.epilog)

Nesting argument groups and mutually exclusive groups is now deprecated (see bpo-22047). Thank you for the bug report.

Note Paul's comment about why nesting mutually exclusive groups does not give you anything in terms of semantics.

At least until these latest deprecations, the only nesting that made sense was to put a mutually_exclusive_group inside an argument_group. This was a way of providing a title and description for the exclusive_group. (And not documented.) I don't know if that's still possible.

'required' only makes sense for the exclusive_group. I don't know what happens when one tries to give it to an argument_group. If it doesn't raise an error, I expect it to be ignored.

argument_groups are only used for help formatting; they have no role in parsing. exclusive_groups are primarily a parsing checking tool. Usage formatting tries to display exclusive groups, but is easily broken.

While mutually_exclusive_group is a subclass of argument_group (and that in turn a subclass of argument_container), very little usage or behavior is inherited. Don't expect any consistency.

A key point, that is easily lost, is that all groups share the _actions list with the parser. When an argument is added a group (either kind), it is, in effect, added to the parser's _actions list. So when parsing, there's only one list of Actions.

A group will also keep the Action in its own _group_actions list, which is used for formatting or for exclusive checking. But otherwise the _group_actions list not used for parsing.

Another point, is that there are 2 default argument_groups. Thus every Action is in an _group_actions list. If an exclusive_group is not nested in an argument_group, its Actions will be added to one of the defaults (optionals or positionals).