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This `argument_values` comes from `_get_values()`. Most of the time is derived from the `argument_strings`. But in a few cases it is set to `action.default`, specifically when the action is an optional postional with an empty `argument_strings`.
test_argparse.TestMutuallyExclusiveOptionalAndPositional is such a case. `badger` is an optional positional in a mutually exclusive group. As such it can be 'present' without really being there (tricky). Positionals are always processed - otherwise it raises an error.
If this is the case, what we need is a more reliable way of knowing whether `_get_values()` is doing this, one that isn't fooled by this small int caching.
We could rewrite the `is not` test as:
 if not argument_strings and action.nargs in ['*','?'] and argument_values is action.default:
 pass # _get_values() has set: argument_values=action.default
 else:
 seen_non_default_actions.add(action)
 ...
is a little better, but still feels like a kludge. Having `_get_values` return a flag that says "I am actually returning action.default" would be clearer, but, I think, too big of a change.