incomplete error testing for nargs

[hpaulj]

Several python issues deal with nargs related error messages:

http://bugs.python.org/issue16970 argparse: bad nargs value raises misleading message

http://bugs.python.org/issue9849 Argparse needs better error handling for nargs

nargs can be one of several strings, e.g. '*', '...' (Const.PARSER) or an integer. But checking for valid values is ad hoc.

One issue is when or where nargs can be 0 or <0

parser.addArgument('foo',{nargs:0})

triggers an error by _StoreAction, saying that nargs should be >0. However

parser.addArgument('foo',{nargs:-1})

is accepted. parser.parseArgs([]) returns { foo: [] }, but formatUsage has problems formatting a metavar (in Javascript, not in the Python).

Another issue is what happens if nargs isn't one of the recognized strings or an integer (e.g. nargs:'1' or nargs:'test'). Neither Python or Javascript raises an error in addArgument. Instead the problems arise in either parseArgs or formatHelp. In both the ultimate issue is whether it can replicate a string nargs times.

In the most recent Python release, addArgument does test whether a tuple metavar matches nargs, and in the process discovers invalid nargs values. But even with this the error messages are not clear. Also parser.addArgument gets this test, but group.addArgument does not.

[rlidwka]

I've ported everything from scratch based on python 3.9 (to be released as argparse version 2.0), it should give out the same errors as python.

This hasn't been fully fixed in python yet. We're gonna port it when they do, but until then please bug python devs to fix it over there.

PS: what does nargs=-1 even mean?