[issue41854] argparse.add_mutually_exclusive_group fails for optional positional arguments

[Reuben Thomas]

Reuben Thomas  added the comment:

Thanks for the hint; could this be documented, please?

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[issue41854] argparse.add_mutually_exclusive_group fails for optional positional arguments

[paul j3]

paul j3  added the comment:

Give the positional a non-none default:

e.g.

group.add_argument('args', metavar='ARGUMENT', nargs='*', default=[],
 help='arguments to PROGRAM')

Since a '*' or '?' positional works with an empty list of arguments, it is 
"always seen".  It requires some special handling to allow it to work in the 
mutually exclusive context.  It's a tricky piece of code that might not be well 
documented (if at all).

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nosy: +paul.j3

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[issue41854] argparse.add_mutually_exclusive_group fails for optional positional arguments

[Reuben Thomas]

New submission from Reuben Thomas :

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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