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I think the correction to the problem that I noted in the previous post is to return 'None, arg_string, None', rather than 'action, arg_string, None' in the case where the action is found in a subparser.
This is what '_parse_optional' does at the end:
 # it was meant to be an optional but there is no such option
 # in this parser (though it might be a valid option in a subparser)
 return None, arg_string, None
An input like '--foo baz' would then produce an 'invalid choice' error. Since '--foo' is an optional that the primary parser does not recognize, 'baz' in interpreted as a positional, in this case an invalid subparser choice.
I'm working on cleaning up a test script.