[New-bugs-announce] [issue41854] argparse.add_mutually_exclusive_group fails for optional positional arguments

[Reuben Thomas]

New submission from Reuben Thomas <rrt at sc3d.org>:

The following code:

group = parser.add_mutually_exclusive_group()
group.add_argument('--install-only', action='store_true',
                    help='just install the program, do not run it')
group.add_argument('args', metavar='ARGUMENT', nargs='*', default=None,
                    help='arguments to PROGRAM')

gives the following error:

    group.add_argument('args', metavar='ARGUMENT', nargs='*',
  File "/usr/lib/python3.8/argparse.py", line 1398, in add_argument
    return self._add_action(action)
  File "/usr/lib/python3.8/argparse.py", line 1621, in _add_action
    raise ValueError(msg)
ValueError: mutually exclusive arguments must be optional

But the 'args' argument *is* optional, as there can be 0 of them.

----------
components: Library (Lib)
messages: 377460
nosy: rrt
priority: normal
severity: normal
status: open
title: argparse.add_mutually_exclusive_group fails for optional positional arguments
versions: Python 3.8

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue41854>
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