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I don't see a valid use case to support "-" in the name of the positional argument.
IMHO, it should raise an error (probably a ValueError) for the add_argument in this case ...
Or we keep it as-is and close as wont-fix: if the op wants to pass "foo-bar" for the name of the positional argument ... it is his problem.
He can retrieve the value if he really want, with something like:
getattr(args, 'foo-bar')
In this case a single note in the documentation about using valid Python identifier for the names could be enough.