Any progress with this? I believe it would fix my use case:

```
import argparse
import pprint

parser = argparse.ArgumentParser()
group = parser.add_mutually_exclusive_group(required=True)

group.add_argument('--device-get-capabilities',
                   action='store_true',
                   help='Execute GetCapabilities action from ONVIF devicemgmt.wsdl')

group.add_argument('--ptz-absolute-move',
                   nargs=3,
                   metavar=('x', 'y', 'z'),
                   help='Execute AbsoluteMove action from ONVIF ptz.wsdl')

group.add_argument('--ptz-get-status',
                   metavar='MEDIA_PROFILE',
                   default='MediaProfile000',
                   help='Execute GetSatus action from ONVIF ptz.wsdl for a media profile (default=%(default)s)')

pprint.pprint(parser.parse_args(['--ptz-get-status']))
```

Outputs (using 3.6.3):

```
 python3 ./test-ex-group-with-defult.py 
usage: test-ex-group-with-defult.py [-h]
                                    (--device-get-capabilities | --ptz-absolute-move x y z | --ptz-get-status MEDIA_PROFILE)
test-ex-group-with-defult.py: error: argument --ptz-get-status: expected one argument
```

Are there know workarounds for this?