[issue9571] argparse: Allow the use of -- to break out of nargs and into subparser

[paul j3]

paul j3 added the comment:

In elsdoerfer's example, the '--' effectively ends the argument list for '--ignore'.  '--' are not allowed in the arguments of an optional (see the end of '_get_nargs_pattern()').

Rather the problem is at the start of _get_values()

        if action.nargs not in [PARSER, REMAINDER]:
            arg_strings = [s for s in arg_strings if s != '--']

'--' is not stripped out of the 'parser' input, hence the error message:

    error: invalid choice: '--'

You can see this by replacing the subparsers with an argument with 'nargs=PARSER'.  The argument will get `['--','COMMAND',...]`.

http://bugs.python.org/issue13922 tries to rework how '--' are handled.  Ideally only the current '--' should be removed, leaving the rest to be handled by the subparser (or whoever else gets the strings).

Looks like the 13922 fix needs another fix, one that removes '--' if it is the 1st string for a REMAINDER or PARSER argument.

With that fix:

    ./script.py --ignore one two -- COMMAND arg1 arg2

should work, assigning ['one','two'] to 'ignore', and ['arg1','arg2'] to 'COMMAND's positional.

(I've tested this in a development version with many other changes.  I'll try to write a simpler patch.)

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If a 'end of a list' flag is still needed (as between 2 * positionals), a 'counter' or 'store_true' optional could be used.  Or a new action class that doesn't write anything to the namespace could be written.

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<http://bugs.python.org/issue9571>
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