id,node_id,number,title,user,state,locked,assignee,milestone,comments,created_at,updated_at,closed_at,author_association,active_lock_reason,draft,pull_request,body,reactions,performed_via_github_app,state_reason,repo,type
436669736,MDU6SXNzdWU0MzY2Njk3MzY=,2916,xr.open_dataset no groups info,6866371,open,0,,,7,2019-04-24T12:32:15Z,2019-07-13T21:42:34Z,,NONE,,,,"#### Code Sample, a copy-pastable example if possible

I have to write some data to disk using xr.to_netcdf() method. The data must be structured in groups so i use the group key word. Reading the .nc file from disk leads to empty data variables if group not supplied. Here some samples
```python
data_ds.to_netcdf(path=savepath,mode='w',format='NETCDF4',group='Audio',engine='netcdf4')

datafromdisk = xr.open_dataset(savepath)
datafromdisk

<xarray.Dataset>
Dimensions:  ()
Data variables:
    *empty*

datafromdisk = xr.open_dataset(savepath,group='Audio')

<xarray.Dataset>
Dimensions:  (time: 15360000)
Coordinates:
  * time     (time) datetime64[ns] 2017-05-30T07:40:00 ... 2017-05-30T07:49:59.992280938
Data variables:
    audio    (time) float32 ...
Attributes:
    unit:     Pa

```
#### Problem description
Actually this is not a real problem, if you keep in mind what data groups are strored in a .nc file. 
At the moment, i work arround with netCDF4 to get infos about possible groups within a .nc file.

```python

from netCDF4 import Dataset
rootgrp = Dataset(savepath)
rootgrp

<class 'netCDF4._netCDF4.Dataset'>
root group (NETCDF4 data model, file format HDF5):
    dimensions(sizes): 
    variables(dimensions): 
    groups: Audio

# or

rootgrp.groups

OrderedDict([('Audio', <class 'netCDF4._netCDF4.Group'>
              group /Audio:
                  unit: Pa
                  dimensions(sizes): time(15360000)
                  variables(dimensions): float32 audio(time), float64 time(time)
                  groups: )])
```

#### Expected Output
Well, at least something like this i would appreciate:
```python
datafromdisk = xr.open_dataset(savepath)
datafromdisk

<class 'netCDF4._netCDF4.Dataset'>
root group (NETCDF4 data model, file format HDF5):
    dimensions(sizes): 
    variables(dimensions): 
    groups: Audio
```

#### Output of ``xr.show_versions()``

<details>
INSTALLED VERSIONS
------------------
commit: None
python: 3.7.1 | packaged by conda-forge | (default, Mar 13 2019, 13:32:59) [MSC v.1900 64 bit (AMD64)]
python-bits: 64
OS: Windows
OS-release: 7
machine: AMD64
processor: Intel64 Family 6 Model 60 Stepping 3, GenuineIntel
byteorder: little
LC_ALL: None
LANG: en
LOCALE: None.None
libhdf5: 1.10.4
libnetcdf: 4.6.2

xarray: 0.12.1
pandas: 0.24.2
numpy: 1.16.2
scipy: 1.2.1
netCDF4: 1.5.0.1
pydap: None
h5netcdf: None
h5py: None
Nio: None
zarr: None
cftime: 1.0.3.4
nc_time_axis: None
PseudonetCDF: None
rasterio: None
cfgrib: None
iris: None
bottleneck: 1.2.1
dask: 1.2.0
distributed: 1.27.0
matplotlib: 3.0.3
cartopy: None
seaborn: None
setuptools: 41.0.0
pip: 19.0.3
conda: None
pytest: None
IPython: 7.4.0
sphinx: 2.0.1

</details>
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