A further complication on this.  With the arguments I defined in the previous post

    p.parse_args('--foo test --baz 257'.split())

gives the mutually exclusive error message.  `sys.argv` does the same.

    p.parse_args(['--foo', 'test', '--baz', '257'])

does not give an error, because here the 'test' argument string is the same as the default 'test'.  So the m_x_g test thinks `--foo' is the default, and does not count as an input.

Usually in testing an argparse setup I use the list and split arguments interchangeably, but this shows they are not equivalent.