[issue18943] argparse: default args in mutually exclusive groups

[Vincas Dargis]

Vincas Dargis <vin...@gmail.com> added the comment:

On 2017-12-06 20:28, paul j3 wrote:
> The default value is used *if the flag is not provided at all.*
> 
> "nargs='?'" provides a third option, assigning the 'const' value *if the flag 
> is used without an argument*.

This did a "click" in my head. It works now with `nargs='?'` and 
`const='MediaProfile000'` as expected, thanks!

I am really sorry for the noise, due to misunderstanding while reading 
(skipping-throuhg?) Python documentation.

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[issue18943] argparse: default args in mutually exclusive groups

[Vincas Dargis]

Vincas Dargis <vin...@gmail.com> added the comment:

On 2017-12-06 19:43, paul j3 wrote:
> With one flag but not its argument, I get the error that you display.  That 
> has nothing to do with the grouping.
> 
> 0932:~/mypy/argdev$ python3 issue18943.py --ptz-get-status
> usage: issue18943.py [-h]
>   (--device-get-capabilities | --ptz-absolute-move x y z 
> | --ptz-get-status MEDIA_PROFILE)
> issue18943.py: error: argument --ptz-get-status: expected one argument

In my example I pasted, I had hardcoded arguments:

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

I expected `python myscript.py --ptz-get-status` to work, because default value 
is set.

I do not compute that "With one flag but not its argument", sorry. It has 
default argument set, shoudn't that work?

Thanks!

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[issue18943] argparse: default args in mutually exclusive groups

[Vincas Dargis]

Vincas Dargis <vin...@gmail.com> added the comment:

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?

--
nosy: +talkless

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