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
294380515,MDU6SXNzdWUyOTQzODA1MTU=,1888,Getting DataArrays from netCDF4 files correctly and without hassle,601177,open,0,,,4,2018-02-05T12:45:42Z,2020-12-06T18:07:47Z,,NONE,,,,"#### Context
Consider a netCDF4 file with a group structure. For example, the following toy:
```python
import netCDF4 as nc
# netCDF4 file
f = nc.Dataset('simple_hierarchy.nc', 'w')
# coordinates in root
f.createDimension('x', 3)
f.createVariable('x', 'f4', ('x',), fill_value=False)
f['x'][:] = [1.1, 2.2, 3.3]
f.createDimension('y', 2)
f.createVariable('y', 'f4', ('y',), fill_value=False)
f['y'][:] = [-0.9, -1.8]
# variables in root
f.createVariable('u', 'i1', (), fill_value=False)
f.createVariable('v', 'u1', ('x','y'), fill_value=False)
# group
f.createGroup('g')
g = f['g']
# new/modified coordinates in g
g.createDimension('y', 3)
g.createVariable('y', 'f4', ('y',), fill_value=False)
g['y'][:] = [-0.9, -1.8, -2.7]
# variable in g
g.createVariable('w', 'u1', ('x', 'y'), fill_value=False)
f.close()
```

#### Current behavior

1. It is currently a hassle to get a DataArray from variable in a group with multiple non-coordinate variables:
    ```python
    >>> xr.open_dataarray('simple_hierarchy.nc')
    …
    ValueError: Given file dataset contains more than one data variable. 
    Please read with xarray.open_dataset and then select the variable you want.
    >>> xr.open_dataarray('simple_hierarchy.nc', group='v')
    xr.open_dataarray('simple_hierarchy.nc', group='v')
    …
    OSError: [Errno group not found: v] 'v'
    >>> xr.open_dataarray('simple_hierarchy.nc', drop_variables='u')
    <xarray.DataArray 'v' (x: 3, y: 2)>
    array([[120, 219],
           [178, 172],
           [  9, 127]], dtype=uint8)
    Coordinates:
      * x        (x) float32 1.1 2.2 3.3
      * y        (y) float32 -0.9 -1.8
    ```

2. Also, coordinates defined at a group level closer tot the root are not taken into account:
    ```python
    >>> xr.open_dataarray('simple_hierarchy.nc', group='g')
    <xarray.DataArray 'w' (x: 3, y: 3)>
    array([[216, 219, 178],
           [172,   9, 127],
           [  0,   0,  64]], dtype=uint8)
    Coordinates:
      * y        (y) float32 -0.9 -1.8 -2.7
    Dimensions without coordinates: x
    ```
    So the DataArray is not loaded correctly, as part of its defining coordinates are missing.

#### Suggested behavior

1. Add a `variable` kwarg in the `open_dataarray` method:
    ```python
    >>> xr.open_dataarray('simple_hierarchy.nc', variable='v')
    <xarray.DataArray 'v' (x: 3, y: 2)>
    array([[120, 219],
           [178, 172],
           [  9, 127]], dtype=uint8)
    Coordinates:
      * x        (x) float32 1.1 2.2 3.3
      * y        (y) float32 -0.9 -1.8
    ```

2. Have the function that loads variables go up the group hierarchy to see if some coordinate arrays can be found for dimensions lacking them within this group:
    ```python
    >>> xr.open_dataarray('simple_hierarchy.nc', group='g')
    <xarray.DataArray 'w' (x: 3, y: 3)>
    array([[216, 219, 178],
           [172,   9, 127],
           [  0,   0,  64]], dtype=uint8)
    Coordinates:
      * x        (x) float32 1.1 2.2 3.3
      * y        (y) float32 -0.9 -1.8 -2.7
    ```
    I guess care needs to be taken as well upon writing to netCDF, to make sure no spurious dimension/coordinate definitions are added.

#### Version

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