existing - chokes if value is a list

    self._check_value(action, value)

proposed change (does not convert strings, but check_value does
    ensure that values are of the correct type)

    if not isinstance(value, list):
        self._check_value(action, value)
    else:
        for v in value:
            self._check_value(action, v)

minimal change - avoids the choke without changing anything else

    if not isinstance(value, list):
        self._check_value(action, value)

closer to other defaults - convert string but don't test it
    use other objects as is

    if isinstance(value, str):
        value = self._get_value(action, value)

closer to ?positional - convert and test string

    if isinstance(value, str):
        value = self._get_value(action, value)
        self._check_value(action, value)

no action - simply use values (passes tests)

    # self._check_value(action, value)

convert and check string; only check list values

    if not isinstance(value, list):
        value = self._get_value(action, value)
        self._check_value(action, value)
    else:
        for v in value:
            self._check_value(action, v)

maximal change (convert and check each value)

    if not isinstance(value, list):
        value = self._get_value(action, value)
        self._check_value(action, value)
    else:
        value = [self.get_value(action, v) for v in value]
        for v in value:
            self._check_value(action, v)

more robust

    if isinstance(value, str):
        value = self._get_value(action, value)
        self._check_value(action, value)
    elif isinstance(value, (tuple, list)): # more generally, iterable
        def foo(v):
            if isinstance(v, str):
                return self._get_value(actin, v)
            else:
                return v
        value = [foo(v) for v in value]
        for v in value:
            self._check_value(action, value)
    else:
        # anything else, just use (or object)
        pass

A variation on _check_value() could take a list, test them individually
and include all failures in the error message

A key point in _get_values
'arg_strings' is always a list of strings, possibly empty
'default' may be string, int, float, list, other object
_get_value, and type expect a string; they may work with other things
(e.g. int(1)), but that is not guaranteed
normal default behavior is to pass string default through type
and simply assign anything else.

The latest delayed default evaluation:
if action is not seen
  default is str
    (further dest tests)
      _get_value(action, default)


These ?* positionals are always 'seen', hence not processed in the 'delayed' block