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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?