issues: 129630652

I came across a situation where my coordinate variable was not dumped as a variable in the output netcdf file using dataset.to_netcdf. In my case I managed to fix it by simply adding variable attributes to this coordinate variable (which didn't have any).

The situation where that happened was while creating a sliced dataset with dataset.isel_points method which automatically defines a new coordinate called points in the sliced dataset. If I dump that dataset as is, the coordinate isn't written as a variable in the netcdf. adding attributes to points however changes that. Here is an example:

``` In [1]: import xarray as xr

In [2]: ds = xr.open_dataset('netcdf_file_with_longitude_and_latitude.nc')

In [3]: ds Out[3]: <xarray.Dataset> Dimensions: (latitude: 576, longitude: 1152, time: 745) Coordinates: * latitude (latitude) float64 -89.76 -89.45 -89.14 -88.83 -88.52 -88.2 ... * longitude (longitude) float64 0.0 0.3125 0.625 0.9375 1.25 1.562 1.875 ... * time (time) datetime64[ns] 1979-01-01 1979-01-01T01:00:00 ... Data variables: ugrd10m (time, latitude, longitude) float64 0.2094 0.25 0.2799 0.3183 ... vgrd10m (time, latitude, longitude) float64 -5.929 -5.918 -5.918 ...

In [4]: ds2 = ds.isel_points(longitude=[0], latitude=[0]).reset_coords()

In [5]: ds2 Out[5]: <xarray.Dataset> Dimensions: (points: 1, time: 745) Coordinates: * time (time) datetime64[ns] 1979-01-01 1979-01-01T01:00:00 ... * points (points) int64 0 Data variables: latitude (points) float64 -89.76 vgrd10m (points, time) float64 -5.929 -6.078 -6.04 -5.958 -5.858 ... ugrd10m (points, time) float64 0.2094 0.109 0.008546 -0.09828 -0.2585 ... longitude (points) float64 0.0

In [6]: ds2['points'].attrs Out[6]: OrderedDict()

In [7]: ds2.to_netcdf('/home/rafael/ncout1.nc')

In [8]: ds2['points'].attrs.update({'standard_name': 'site'})

In [9]: ds2['points'].attrs Out[9]: OrderedDict([('standard_name', 'site')])

In [10]: ds2.to_netcdf('/home/rafael/ncout2.nc') ```

Here is the ncdump output for these two files:

$ ncdump -h /home/rafael/ncout1.nc netcdf ncout1 { dimensions: time = 745 ; points = 1 ; variables: double time(time) ; time:_FillValue = 9.999e+20 ; string time:long_name = "verification time generated by wgrib2 function verftime()" ; time:reference_time = 283996800. ; time:reference_time_type = 0 ; string time:reference_date = "1979.01.01 00:00:00 UTC" ; string time:reference_time_description = "kind of product unclear, reference date is variable, min found reference date is given" ; string time:time_step_setting = "auto" ; time:time_step = 3600. ; string time:units = "seconds since 1970-01-01" ; time:calendar = "proleptic_gregorian" ; double latitude(points) ; string latitude:units = "degrees_north" ; string latitude:long_name = "latitude" ; double vgrd10m(points, time) ; string vgrd10m:short_name = "vgrd10m" ; string vgrd10m:long_name = "V-Component of Wind" ; string vgrd10m:level = "10 m above ground" ; string vgrd10m:units = "m/s" ; double ugrd10m(points, time) ; string ugrd10m:short_name = "ugrd10m" ; string ugrd10m:long_name = "U-Component of Wind" ; string ugrd10m:level = "10 m above ground" ; string ugrd10m:units = "m/s" ; double longitude(points) ; string longitude:units = "degrees_east" ; string longitude:long_name = "longitude" ; }

$ ncdump -h /home/rafael/ncout2.nc netcdf ncout2 { dimensions: time = 745 ; points = 1 ; variables: double time(time) ; time:_FillValue = 9.999e+20 ; string time:long_name = "verification time generated by wgrib2 function verftime()" ; time:reference_time = 283996800. ; time:reference_time_type = 0 ; string time:reference_date = "1979.01.01 00:00:00 UTC" ; string time:reference_time_description = "kind of product unclear, reference date is variable, min found reference date is given" ; string time:time_step_setting = "auto" ; time:time_step = 3600. ; string time:units = "seconds since 1970-01-01" ; time:calendar = "proleptic_gregorian" ; double latitude(points) ; string latitude:units = "degrees_north" ; string latitude:long_name = "latitude" ; double vgrd10m(points, time) ; string vgrd10m:short_name = "vgrd10m" ; string vgrd10m:long_name = "V-Component of Wind" ; string vgrd10m:level = "10 m above ground" ; string vgrd10m:units = "m/s" ; double ugrd10m(points, time) ; string ugrd10m:short_name = "ugrd10m" ; string ugrd10m:long_name = "U-Component of Wind" ; string ugrd10m:level = "10 m above ground" ; string ugrd10m:units = "m/s" ; double longitude(points) ; string longitude:units = "degrees_east" ; string longitude:long_name = "longitude" ; int64 points(points) ; points:standard_name = "site" ; }