[issue46058] argparse: arg groups and mutually exclusive groups behave inconsitently

[paul j3]

paul j3  added the comment:

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).

--
nosy: +paul.j3

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[issue46058] argparse: arg groups and mutually exclusive groups behave inconsitently

[Irit Katriel]

Irit Katriel  added the comment:

Nesting argument groups and mutually exclusive groups is now deprecated (see 
issue22047). 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.

--
nosy: +iritkatriel
resolution:  -> duplicate
stage:  -> resolved
status: open -> closed
superseder:  -> Deprecate unsupported nesting of argparse groups

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[issue46058] argparse: arg groups and mutually exclusive groups behave inconsitently

[Irit Katriel]

Change by Irit Katriel :


--
versions:  -Python 3.6, Python 3.7, Python 3.8

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[issue46058] argparse: arg groups and mutually exclusive groups behave inconsitently

[László Attila Tóth]

László Attila Tóth  added the comment:

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)

--

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[issue46058] argparse: arg groups and mutually exclusive groups behave inconsitently

[László Attila Tóth]

New submission from László Attila Tóth :

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 #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 #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 #2 and #3 .

--
components: Library (Lib)
messages: 408405
nosy: Laszlo.Attila.Toth
priority: normal
severity: normal
status: open
title: argparse: arg groups and mutually exclusive groups behave inconsitently
type: behavior
versions: Python 3.10, Python 3.11, Python 3.6, Python 3.7, Python 3.8, Python 
3.9

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