issues: 366626025

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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
366626025 MDU6SXNzdWUzNjY2MjYwMjU= 2461 Set one-dimensional data variable as dimension coordinate? 43126798 closed 0     13 2018-10-04T05:19:25Z 2019-11-21T23:57:15Z 2018-10-04T06:22:03Z CONTRIBUTOR      

Code Sample

I have this dataset, and I'd like to make it indexable by time:

python <xarray.Dataset> Dimensions: (station_observations: 46862) Dimensions without coordinates: station_observations Data variables: time (station_observations) datetime64[ns] ... SNOW_ON_THE_GROUND (station_observations) float64 ... ONE_DAY_SNOW (station_observations) float64 ... ONE_DAY_RAIN (station_observations) float64 ... ONE_DAY_PRECIPITATION (station_observations) float64 ... MIN_TEMP (station_observations) float64 ... MAX_TEMP (station_observations) float64 ... Attributes: elevation: 15.0

Problem description

I expected to be able to use ds.set_coords to make the time variable an indexable coordinate. The variable IS converted to a coordinate, but it is not a dimension coordinate, so I can't index with it. I can use assign_coords(station_observations=ds.time) to make station_observations indexable by time, but then the name in semantically wrong, and the time variable still exists, which makes the code harder to maintain.

Expected Output

```python ds.set_coords('time', inplace=True) <xarray.Dataset> Dimensions: (station_observations: 46862) Coordinates: time (station_observations) datetime64[ns] ... Dimensions without coordinates: station_observations Data variables: SNOW_ON_THE_GROUND (station_observations) float64 ... ONE_DAY_SNOW (station_observations) float64 ... ONE_DAY_RAIN (station_observations) float64 ... ONE_DAY_PRECIPITATION (station_observations) float64 ... MIN_TEMP (station_observations) float64 ... MAX_TEMP (station_observations) float64 ... Attributes: elevation: 15.0

In [95]: ds.sel(time='1896') ValueError: dimensions or multi-index levels ['time'] do not exist ```

with assign_coords:

```python In [97]: ds=ds.assign_coords(station_observations=ds.time)

In [98]: ds.sel(station_observations='1896') Out[98]: <xarray.Dataset> Dimensions: (station_observations: 366) Coordinates: * station_observations (station_observations) datetime64[ns] 1896-01-01 ... Data variables: time (station_observations) datetime64[ns] ... SNOW_ON_THE_GROUND (station_observations) float64 ... ONE_DAY_SNOW (station_observations) float64 ... ONE_DAY_RAIN (station_observations) float64 ... ONE_DAY_PRECIPITATION (station_observations) float64 ... MIN_TEMP (station_observations) float64 ... MAX_TEMP (station_observations) float64 ... Attributes: elevation: 15.0 ``` works correctly, but looks ugly. It would be nice if the time variable could be assigned as a dimension directly. I can drop the time variable and rename the station_observations, but it's a little annoying to do so.

Output of xr.show_versions()

INSTALLED VERSIONS ------------------ commit: None python: 3.6.6.final.0 python-bits: 64 OS: Linux OS-release: 4.16.0-041600-generic machine: x86_64 processor: x86_64 byteorder: little LC_ALL: None LANG: en_AU.UTF-8 LOCALE: en_AU.UTF-8 xarray: 0.10.2 pandas: 0.22.0 numpy: 1.13.3 scipy: 0.19.1 netCDF4: 1.3.1 h5netcdf: None h5py: None Nio: None zarr: None bottleneck: 1.2.0 cyordereddict: None dask: 0.16.0 distributed: None matplotlib: 2.1.1 cartopy: None seaborn: None setuptools: 39.0.1 pip: 9.0.1 conda: None pytest: None IPython: 5.5.0 sphinx: None 
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